It states at the bottom of an answers page that you should vote up if an answer adds to the discussion, or vote it down if it does not. So that's the general position.
So it's really a judgement call for each individual. From what i've noticed answers tend to most often get voted down if they are innapproriate or in some way objectionable to others.
Sometimes the types of comments that you may disagree with can get more comments/answers coming in so you could argue they add to the discussion/answers (inadvertently at least).But i suppose this is a place primarily for answers per se rather than discussion as with forums hence you can only contribute once.
If it contributes in your opinion - Vote Up
If you think it would make no difference if it was deleted -Vote Down
I imagine a some people vote up whenever they agree regardless of what it necessarily adds to the answering of the question. We all like to think we're right after all (Mostly anyway)