After reading this I realize that I still vote up comments I agree with rather than comments that add value to the discussion. Why is it so hard to change your behavior in this regard?
replies(1):
1) I was going to post the same comment. I'll just upvote him instead.
2) I had never thought of that. It makes sense and I agree. I'll upvote him.
Both cases are "I agree". But for 1) If I was going to post the same comment, then I clearly see it as something relevant to say in the discussion. Therefore his post is relevant and should be upvoted. 2) is also a case of something being relevant.
Should I also be spending my time finding things I disagree with but find insightful and upvoting them? Insightful generally falls under category 2.
Basically, agreement and value go hand in hand, and as long as you aren't treating comments like a poll, most if not all of your upvotes should be the result of you agreeing with the comment.