I wanted to add to Stormy's point and say that it's NOT down-voting when you are honestly voting that way. In my opinion, down-voting was coined to refer to people purposefully voting lower than is justified in an attempt to lower someone's rank, or retaliate, something like that. I understand though that there is a concern about retaliation, but I always tell new people that if you give someone a higher score than they deserve, then you are practically giving them free assignments for which they will write poor reviews and since that incenses most people...

Anyway, as long as you can justify the vote, there is no problem. Now, if you suspect you are being retaliated against, there is recourse -- just mention it to Stormy and she will check it out. Granted, it would have to be obvious to bust someone, however, a warning could also go a long way towards stopping it. Finally, I know a lot of people just refrain from voting rather than be seen as giving low ratings and since that doesn't help (or hurt) the reviewer's rank, I think it's a better option.