Ask HN: Do you upvote? Why or why not?

I recently realized that I rarely upvote or downvote so I decided to be more of a good community citizen and start doing so. Then came the question of what does an upvote or downvote mean? Do I downvote an opinion I disagree with even though it is interesting and well reasoned? (By some reasoning, perhaps I should actually be upvoting such comments). I'm obviously overthinking here but I'm curious so indulge me. What is your heuristic for up/down-voting?