If a post is official, it should have good spelling and grammar. Otherwise, it won't look professional. If English isn't the first language of an admin, it's going to be more difficult, but I'd advise putting it through a spell checker, or asking mod staff to look over things like rules before posting them.
Personally, I don't sign up to sites where the spelling and grammar is bad. I expect that other people will react the same way, so I think first impressions are very important.
My spelling and grammar is very bad, well not as bad as my father who I don't know. I also had it and I have to try and learn and try my best. It's hard since I always had a learning disabilities.
So think to yourself that I have that and read each of my posts?
It's just a reality though that important topics need to have good spelling and grammar, and it's going to affect how seriously people take you. Not trying to be a meanie... that was legit advice. You have to be honest about how a person can improve their signups. If you're worried about it, get a friend to proof read anything that is going out to the community such as forum announcements and rules. Do it! lol. Your personal posts in topics don't matter quite as much. It could honestly be the difference between getting a member to sign up and them not signing up.
They're not going to scrutinize you @Nebulous, but they are probably going to think "this guy isn't serious." They won't spend enough time there to do that. Even if your forum isn't a business, you've got to think of it along that line. You probably have a 30 second window to impress that guest on your forum. Would you take a company seriously if an official email to you had bad grammar? No need to answer that, just think of it honestly, and there is your answer. 😉
I think they matter. It is not about never making a mistake, we all will at some point; it is only human. It is more about to no intentionally ignore punctuation and alienate your potential readers, which kind of beats the point of sharing your thoughts with the community.
That said, I don't mind having a thread to speak in 1337 and stuff just for the fun of it.
I definitely think proper spelling and grammar matters as it reflects you personally and your forum. It's part of the initial impression guests/visitors get when visiting your forum for the first time. Constant spelling and grammar mistakes affect the readability of content and just makes it hard to read posts/threads overall. It doesn't take that much time to spell check your posts or even look up the word online if you're unsure. On my forum we sometimes let each other know when there's been a spelling/grammar mistake large enough that will affect the context of what is written; everyone appreciates it (the forum categories where this happens is more "official", though). No one's perfect so there will be mistakes and that's to be expected.