No, but it makes your posts easier to read. And like others, I usually ignore posts that are misspelled and ungrammatical. There's lots of good information on this site and everybody has a something to contribute. But it would be a shame to post some good stuff only to have people ignore your post because you can't be bothered to try spelling things correctly.

On the same topic: What's really annoying is a long post that's a single paragraph. Does it take so much effort to hit the return key every once in awhile?

Edit: Dang it. Here I go on a spelling rant and I just caught one of my own!