Separately, as a karma strategy, it makes sense to upvote something that you reply to (regardless of whether you agree or disagree) as it tends to make your reply higher up (because the parent comment is higher up) and get more visibility which in theory should result in more karma.
I do this, but I justify it by thinking that the thread was important enough to me to reply to, so I should vote. I don't care about karma, but I'm more likely to get replies, or so I hope.
Separately, as a karma strategy, it makes sense to upvote something that you reply to (regardless of whether you agree or disagree) as it tends to make your reply higher up (because the parent comment is higher up) and get more visibility which in theory should result in more karma.