My view is that more important is that rewards stay on steem blockchain and they are not cashed out. But even in that case somebody else will buy it and bring steem back. If I upvote myself my steem power go up and when later on upvote somebody else his reward is bigger to. Or if I upvote just my friend. His upvote is worth more and he can reward other people more. So long term I think that this will not be a problem. Upvote you self doesn't look good but have little impact on whole steemit community. My thoughts anyway. But I could be wrong. I don't know.