This is more directed towards admins of small forums, rather than large ones. If you find a blatant spelling or grammar error in a thread title or post, do you edit the title/post and correct it, or just let it be? I'm speaking from an admin's perspective, and I'm not referring to your own posts but posts from other users on the forum.
Maybe I'm too much of a perfectionist but I'm quite conflicted on this subject.
Talking about small, with a relatively small userbase and active userbase? Depends If it bugs me, due to not matching the topic at hand, I might fix it. If it is just general spelling errors, I most likely will tell the user about the oops in spelling or grammar.
For the title I will correct the error most times but not all. Some errors just reflect badly on the site itself but I don't mess with spelling errors in the posts themselves.