I've always deleted the pre-made hoods because I just wanted to play with my own Sims and households. Now that it's been two years since I've played, I'm wondering if that was in fact a terrible idea, and I can't find anything that tells me either way. Will it fireball my game? Or is it safe as long as I haven't played them?

It is safe regardless of if you played with them or not. It is safe to delete a whole Neighborhood from within the game.
What is not safe, is to delete sims within a Neighborhood, because it leaves unwanted crumbs all over that Neighborhood. Deleting the whole thing well..there is no crumbs to leave behind!

Thank you very much

Delete away. We have a great thread for small questions like this here: http://modthesims.info/showthread.php?t=574020

I haven't had those neighbourhoods in my game for over 15 years.

I don't completely agree. Belladonna Cove, Riverblossoms Hill and Desiderata valley are good to delete at once.

Regarding the other three : although the above posters are all correct - it wll not harm your game at all - until the day you may want to put them back. Pleasantview, Veronaville and Strangetown has the hood numbers N001, N002 and N003 (among the three anyway).
Your game will name a custom hood - if you make it - N001, the next one N002, and the next one N003.
So imagine you want to put Pleasantview back for whatever reason under the sun - it will merge with your custom hood and both will become unplayable.
I think it is fair to tell players this - because I ended up doing exactly this and lost a hood this way. The reason why I wanted to add Pleasantview back was due to taking part in a competition and I had trouble, using UC, to find base game only items.
Because when I deleted them in the first place, I was told that it is safe - but not that it could create problems later if I should want them back.

So it would be interesting to know whether it's okay to delete them after creating a custom hood (N004) If you delete them after having a N004 and then create more neighbourhoods, does the game number them N005, N006 and so long, or does it jump back from N004, and names them N001, N002 etc?

Guys...there are renaming program that renames in batch. I use Ant Renamer, and there are more. If your current 'Hood was named N002 by the game, and because reasons you'd like to re-add the original N002 Neighborhood, well, you have two choices. Either batch rename the original, or batch rename yours. It take seconds to do.

No, it does not - the game will create a N001 hood first, @Orphalesion
My post is, indeed, about what can happen, not what will happen.
And a player is still free to delete those, but should know that just replacing it would cause her game to be gone forever.
Personally, I have Jawusa's 3 empty hoods in my game.
@Rosebine - yes, that is true - but I also found that AFTER I did what I did. A player can decide to rename her hood. Back then,I knew very little about the game's workings at that time and I think it is unreasonable to assume everyone does. I was very fortunate that I did not mess up my main and most beloved hood, but I could have.
Because nobody told me !

You can get around that by putting in empty folders called N001, N002 and so on and the game will not use those numbers.

Good suggestion by marka93, it sounds like the easiest option to me, I have to say.
I will keep that in mind if I see questions in the future about this.

I never seriously thought about those first three 'hoods before, I deleted them from my game many, many years ago and it is certainly safe to do so. However I do start a fresh The Sims 2 folder for testing quite regularly and so use those 'hoods for testing.
When the game first creates The Sims 2 folder the Storytelling for those three 'hoods only is placed in the main Storytelling folder and the parts for each 'hood are moved into each 'hood's Storytelling folder when you first open the 'hood. Once all three have been initialized the main Storytelling folder is cleaned out. Would it be left full of junk if those 'hoods were deleted without ever having run?

The game scripting for them depends on their neighbourhood ID number. if they were re-added later that neighbourhood ID would change so the game scripting would not only not run on them but probably try to run in your first custom 'hood. Game scripting checks sim's GUID so you wouldn't actually see it run but it would try if a 'hood has an ID of 1,2 or 3.

The hood was not really usable after that and I did some googling and found somewhere (it is a while ago) that it would not be, @Chris Hatch.
I really did not know much about anything except about how to play the vanilla game back then - my life was very different then and if it did not change drastically,the chances are that I might not have decided to enter a contest and it would never have happened. Well, to be fair, I still don't know a lot
But I do know that I lost a hood by doing this and I hope that it does not happen to anyone else.

Honestly if you are going to add back a pleasant view I would just create it's own folder and move the custom content there in downloads. Frankly I prefer one hood per folder (except for testing hoods but then they are throwaways anyway). The game seems to run better with a lot of cc if you just have one hood

Seriously, does the game run faster with less hoods?

Well, there's nothing in my storytelling folder. I recently installed the game on the Win7 side of my computer, immediately deleted everything EAxis - the neighbourhoods - and put my own neighbourhoods in there. Didn't even think about the Storytelling folder so it looks as though the game deletes the files if you remove the neighbourhood. I have a N001 too - it's one of my testing hoods.

Would another option be to take your custom hood out then delete a neighbourhood? For example if I want to delete my borked Pleasantview and replace it with a clean version - take out my custom hood, delete, add clean one then put custom hoods back?

As long as the numbers aren't the same yavannatw, you can do what you like. What you shouldn't do is allow files of one neighbourhood to overwrite files of another neighbourhood with the same number. That's the express to borkdom. If you have two neighbourhoods with the same number, you can have both by renaming all the files in one neighbourhood to a unique number. So, say I had an N002 and wanted to replace Strangetown (never gonna happen but still). I'd need to take my custom neighbourhood (or indeed the copy of Strangetown if I preferred) and rename every single instance of the neighbourhood number (N002) to something else - say N012. Fine as long as I have no N012. Doing it by hand is tedious (in an initialised and played hood) so I'd use a file renamer programme. There are plenty on the web for free. You can hurl neighbourhood folders in and out of the main neighbourhood folder in any order you like. The game reads what's there (as long as the files have the right names). You do occasionally have to delete the neighbourhoodmanager file. I wondered if you thought the game renumbered the files - it doesn't. Each neighbourhood folder has to have all files and folders inside it with the same number for the game to see it properly and for it to show up in game.

Neighbourhood folders are your saved games - I would guess it's obvious that if you overwrite one with another, you are going to make a mess of things or at least lose one of them. Though the reality is you lose both.

It is nevertheless a mistake I made, because it was not obvious to me at that time. And it is a mistake that could be made by others, so I just think a person who asks about it, should know about it.