It depends, FiveHoursSleep. Patch will sit beautifully if I stop to talk to someone. Then he quietly chews right through his lead . We had to have a chain lead in between the collar and his actual lead. Now we use a harness it's better, because it's out of the way.
i think it's okay for them to do this isn't it? I've always encouraged my dogs to play with their leads. If it's too excessive get him to do a bit of heel work, or some sits or downs to get him in a 'working' place. Or maybe if he's a retrieving breed, he'd carry a ball or retrieving toy?