Leatherstocking: Mouthing, chewing or biting (even softly) is a sign of dominance or possession, especially in puppies that need to establish themselves. Small children often fall victim to this, as they are small and defenseless in the dog's mind, and may suffer a few light bites or even skin punctures as a result of the dog being firm, possessive or dominant with them.
The Dog Whisperer show on the cable network's National Geographic channel deals with this behavior quite often as it is common in puppies and even larger dogs. Their position is that if the animal is demostrating a behavior you do not want to continue, cause the animal to stop and initiate another, more acceptable behavior. This is a show well worth watching if you haven't seen it yet.
For example: when my 3 y/o 90lb Bouvier des Flandres starts mouthing or chewing on my hand I say 'No'! Then I command him to 'Sit', and command him to 'kiss'. The 'Sit' and 'Kiss' commands are for the acceptable behavior, which is whatcha want. The focus here is that an unwanted behavior is corrected with a command for the animal to do something else in order to get its attention off or away from its dominant behavior and to make it understand it is subordinate to you, the Master.......... Then if you wish you can reward the dog for its proper, submissive behavior (sit, kiss, lay down, etc.)
Do not be afraid to use the 'No' word. They understand, even as puppies. Even labs understand that and they like to chew.... HTH. Mikey.