I downvote obvious fallacies and bad faith. And people who fan the flames in a thread. I don’t downvote posts because I disagree with them, but that’s just me. (I don’t upvote because I agree either)

Overall, it is common to see greyed posts downvoted for questionable reasons, but I don’t think I’ve ever seen a dead post that did not deserve it. So overall it seems to work and I avoid overthinking it.