Redundant. I only downvote posts that I think are outright flames, or otherwise completely counterproductive to conversation. Downvoting is not a way of saying "I disagree with you," it's a way of saying "You're detracting from real conversation." Explaining downvotes only adds to the noise.

I do, however, upvote to agree. If I disagree but I think it's a legitimate comment, I don't do anything.