Good spelling matters a lot in some contexts, and not a lot in others. On a message board like this, I tend to assume that people are typing quickly, in order to keep a 'conversation' flowing, so mistakes will creep in - and as others have said, I can tell what they are saying, and that's what matters here.
But spelling mistakes on important documents show a lack of attention to detail, and I do think this makes a very poor impression.
I did actually correct someone's spelling on one of the threads about tomorrow's teachers' strike - where a NE London tutoring college has posted an advert for their establishment, where they offer 'tuitioning', and misspell successful, as well as misplacing an apostrophe. For me, it seems inexplicable that an educational establishment would seek to advertise its services, with a mistake-ridden advert!
I don't think I am superior because I can spell well - but I don't see why I should apologise for this ability either. There are plenty of things that I am bad at, so why shouldn't I be proud of something I am good at?