If it contributed to the conversation, I upvote. Especially if it’s well-written and informative.
If it’s off-topic, incorrect, or repetitive, I downvote.
If it’s banal or uninteresting but not actually deleterious to discourse or distracting from the topic, I don’t vote.
I upvote almost all replies to my comments, including those disagreeing with me, unless they clearly didn’t take the time to read and understand my comment.
I vote on almost all comments I reply to, as well.