Nibbling

I would stop the behavior. It is a good thing that she is being gentle about it, but its a bad habit for a dog to nibble- especially a large or "scary" dog. If she does it with strangers, it can be misinterpreted as aggression biting and of course, even with you she might mess up your clothing or accidentally scrape you.

there are several stickies and mentions in threads; read 'the bite stops here" and search for "bite inhibition"

One tactic is to encourage the dog to lick you to show her playfulness rather than nibble. She still gets to have that contact with you but in a gentler, safer manner.

I agree with Shell - be careful about nibbling. Pits have a 'grooming' type of behavior that I would describe as nibbling, and seems to tickle, rather than be like nipping. It is very different than nipping clothes. So here's a suggestion:
1. You must teach a Lab/Pit Bite Inhibition, b/c either breed can do significant damage during play when they get excited - kids scream and run around, they get bitten by an excited dog, owners get sued, dog gets put down.
2. However, if you understand the process, you can try to discriminate and focus on all biting, nipping, pulling, except for the nibbling. But any time the dog gets excited and nibbling turns to nipping, you have to stop that.
3. Even when gentle, soft-mouthed dog, the strong recommendation for Bite Inhibition is b/c an injured, surprise, or excited can bite down to the bone... however, a dog with Bite Inhibition will rarely break the skin, even in emergency situations....