As I see it, "upvote = agree" increases the danger of groupthink. I try to downvote complete wastes of time or detractions from the discussion, and upvote thought-provoking contributions even when I disagree.
groupthink is a state where people self-censor in fear of rejection or repression.
Only down-voting when disagreeing can cause groupthink since people who disagree with the consensus will stop posting disagreeing comments to preserve their karma.