Retrievers do hold things in their mouths at certain times.
It's funny. Both Roxy and Daisy do this. When I toss the frisbee-if they need to tinkle they'll do it w/ the frisbee in their mouths. It's a control/comfort thing. Rocky, a golden i know well, always has to have a tennis ball to control. But he never retrieves-like handing it off, to a human. If left unchecked, and without much attention from humans, otherwise, they can get very neurotic w/ those balls.
I'm a bit intolerant about teeth, so i wouldn't allow the gentle wrist biting thing. But then, my dog has a dominant personality. If the dog was always gentle otherwise, i wouldn't worry.