Most of the time, I can't type as fast as I am thinking and just need to get it down before I lose it. Most of the time I run the minimal spell checker in Blogger, make those corrections, then do a once over and make changes. However, that is not always enough to be sure that all of the err's are unerr'ed.
Three are times where at some point after a post is published that I read it and recognize errors, then fix them.
I just can't be the guy that only publishes to the blog when it's perfect though.