I think you're right, I've seen that happen recently. I don't think there's an official rule, but I'd rather there be some sort of limit. I can see a sort of "poster's remorse," where you post something and think, "oh, crud, I shouldn't have said that" and delete immediately. It'd be nice if there was a point after which you can't delete. At that point, I think it's best to click "modify" on the post, even to replace what you've written with something like "deleted because I shouldn't have posted while I was in a bad mood." That'd be my preference, I realize others may feel differently.