I admit I stopped marking a while back. I have noticed however, the better DPS (and I suppose healer if you going to do some damage too) will start on the target I Provoke / pulled, or ranged toss pulled first. And then they usually then down the next "dangerous" one in the pack (usually the one dropping AOE or something). All without marking. Unless it is a lower level dungeon where especially someone is new I will still mark, or if I am waiting on a patroller I will mark that so people may know why I am just standing there.