her_Joe I have one chance to "sell" my words and ideas to the reader ... and I wouldn't show up on a sales call with my fly unzipped or in a dirty shirt. That kind of carelessness tells the "buyer" that I don't respect them. I want to flatter the reader so s/he knows that I care what s/he thinks and how s/he responds. So I check my spelling. I check my grammar. I check the formatting.
That doesn't mean that I don't make mistakes. Many slip by. But I've found that I can catch most of them by reading the story in reverse, one sentence at a time. I can catch them by reading paragraphs out of sequence. And I can catch them by sending them to my editor, a patient woman who goes ballistic when I do something stupid to disrupt my characters or logic. I edit hers for her, and she does mine for me.