I think that if I was upvoting my own comments, there are people who upvote my posts that might stop upvoting my posts because some people consider upvoting your own comments to be unacceptable behavior on steemit. That seems to be the more commonly held belief on steemit. There is also the possibility that someone might down vote my comment because I upvoted my own comment.
I upvote my posts last simply because, for me, it seems to be the right thing to do. I always upvote my own posts at 100 % because why not. I value the people who upvote my posts, and that is just a little thing that I can do to thank them in a small way. Most people probably are not even aware that I do this, but that's ok too.