pups NEED to bite and mouth each other, adult dogs and people, and this sublimates as they mature. to tell a pup not to bite you is like telling a toddler not to try to walk. i always have a little toy in my pocket and let the pup have a chew on that if he wants to, while i hold the other end.
mouthing in an adult dog is not always dominance related, in fact there is a whole pattern of dominance behaviour of which biting is one small aspect, and if a person hasnt been able to read all the other signs before reaching the bite then they really shouldnt have a dog.
skycat is right, highl