pg has said many times that it's OK to downvote out of disagreement (particularly since it's OK to upvote due to agreement.)

I generally don't unless the disagreement has another value-subtracting component (lack of substance, poor reasoning/phrasing, factual errors, unnecessary incivility.) For disagreeable but worthwhile comments, I prefer responding instead. But I recognize the value in having differences of opinion over the use of downvotes -- comments of marginal quality (which some people disagree with, and others don't find valuable enough to upvote even if they agree) will end up dropping down the page.