Like all manners, there are times when it is appropriate and times when it is not. You simply correct the puppy when it is not by making him release his bite and scold him to not do that. For other occasions, such as a rough wrestle, it can be quite appropriate for the dog to have an open mouth for you to grab the dog's mouth and shake, but the dog will not bite. For one thing, if he does, the fun stops. Once you reach that level, you should let the dog win on occasion and at least tie most of the time.