When someone honestly presents their position, and is open to discussing it further, there is no need to down vote their comment for being wrong. In fact, it is counterproductive. By discouraging them from expressing their incorrect position, you do not cause them to relinquish it. By instead explaining why you think it is wrong, you help them to adopt a better position. And if it happens that they were right and you were wrong, then you have the opportunity to learn something.

I tend to down vote comments that are off topic, incoherent, arrogant, or present a conclusion without support.

I tend to up vote comments when they are eloquent, insightful, and correct, or sometimes, when they say pretty much what I was planning to say.