Spelling and grammar errors just grate me a little; and unfortunately, it does call into question the intelligence of the person making those mistakes. I know that's not fair, but it's really hard to take a post seriously when it is riddled with mistakes.
I'm not the smartest guy on the planet (by far!) and intellectual snobs grate on me even more than spelling errors, so I try not to blatantly correct others when they make those mistakes...however, I will try to do so in a subtle manner (for example, in my response, I will repeat the word, only with the correct spelling). I try to approach it from a perspective of teaching, instead of pointing out that the writer has made a mistake in an attempt to make them look stupid.