I've found that if you've done enough reading and writing, it's hard to get away from grammar and spelling. It's like, could I really write uncapitalized sentences...? It'd be a chore for me; I'd have to concentrate on doing it.
So, when I find a post with no sense of grammar/spelling--and I mean more than just occasional mistakes--it leads me to assume that it's the work of someone who isn't all that into reading and writing, etc. Does that impact how I initially view the post? Absolutely it does.