When the customer says it is. There is a snotty attitude that some chefs and waiters have about well-done meat. It seems to be something younger, insecure people espouse. I have certainly been guilty of it myself and I'm sure from time to time it rears its head with me but really, what do I care how someone else likes their food? I mean, I don't have to eat it.
I do make recommendations for specific items, like venison (tough past medium rare-medium), tuna (dryer the longer you cook it) but mostly because I don't want to pay for my customers ideas. In other words, if you want your venison cooked well, fine. But if it's dry and a bit tough that's part of the bargain. You pay for it.
Why do I bring this up? Tonight a waiter put a modifier on one of the tickets referring to a customer as an a-hole because he ordered tuna well done. He may indeed have been an a-hole, but perhaps for other reasons. Not because he wants his food cooked the way he wants it.