On HN I almost never downvote. On reddit this was different. Not sure why but I feel that on HN karma is so much harder to get that I don't want to be an obstacle. On reddit I could more easily downvote. It also was easier to get karma. (Reddit is annoying with regards to its censorship though. That is one arbitrary censorship over there ...)
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.