As a dd I've been on both sides of it, any dd who says they have never gone off target occasionally are full of it.
In regards to target marking, it is a waste of time aside from maybe attaching m1 to your toss to call that one out more past 30-40. Good dd are going to be watching the threat bars and how fast each mobs hp is going down. Bad/don't care dd are going to ignor them anyway.