I would not
encourage a puppy to mouth your hands or any other body part (that is just confusing, when you later try to phase it out). But at the same time, I certainly would NOT pre-empt his learning process by just mindlessly stopping him from using his mouth altogether. If you were to do that, you would fail completely to teach him about bite inhibition (how to use his mouth gently) and end up with a dog who - when the situation arises sometime in his life that a person inadvertantly causes him pain or fright, as is near certain to happen - will bite HARD, since he wasn't taught about the fragility of human skin and the need to be gentle.
Puppies bite - and for a very good reason. They have needle-sharp little daggers for teeth, not because they're any good for eating, but precisely because they will HURT when the puppy bites, even with very little pressure. That fact causes his littermates, or anyone who comes into contact with his teeth, to shriek in pain if he exerts any pressure. This feedback in turn teaches the puppy to be gentle with his jaws. It is absolutely imperative that you go through this learning process with your puppy - because if you don't, he will never learn that even a small amount of pressure hurts humans and he needs to be extremely gentle. And if he
doesn't learn that - then the day someone inadvertantly slams some body part in a door, stands on him in heels, or some small child fall on him - then he's going to react with a "nip" that is likely to be a good hard bite, since he knows nothing about how to be gentle. I think you can probably see already that bite inhibition is actually the most important lesson in the life of a dog that is going to live with people, especially if it is going to be around children
So how does this apply to your actual question? Well, offering up body parts to be chewed upon by a teething puppy is not necessarily a good idea. I certainly would not suggest that you allow your hands to be a chew toy. HOWEVER, just stopping a puppy who wants to bite or chew upon you from doing so is NOT the right answer either. Rather, you should use the opportunity for training the pup about bite inhibition (let him know it hurts, and gradually encourage him to be more and more gentle by shrieking in pain at lower and lower pressure levels, till he starts to understand. ONLY then should you start to phase out the biting altogether). And at the same time, invest in something appropriate - like a raw bone or non-edible nylabone - for him to take his real teething frustrations out on.