If I recall... in a dominant dog it can be a sign that the dog is trying to dominate you.... I let mine do it because I like that they have 'bite inhibition' or knowing not to bite hard, rather than not biting at all.
I had to teach our Tinkerbell as an adult, and I had my doubts she would ever learn. I suspect this is what landed her at the Humane Society twice! She's a good girl now and I pretty much trust her not to bite hard. She's not real reliable with the 'under the covers' game, but we're working on that. She just gets overexcited sometimes.
Here's a good article about it:
Puppy Socialization and Bite Inhibition | Modern Dog magazine - the best dog magazine ever