I've also heard mostly that it is a submissive sign of affection and respect, like a puppy would do to its mother. My dad always said it was to get our salt, ew. My training book said it's a sign of nervousness when training. Eddy gets SOOO licky when he's in trouble, haha.
I come home from running on a hot day, and Al licks the sweat off my face, arms, body. Egotistically, I thought this an endearing display of affection, but when I considered the other things these animals love to sniff, lick, and taste, I began to find this practice less flattering.
I think it has a lot to do with how you train. I personally substituted biting with licking, in turn he perceives this as a good thing, I don't really care so its not a bad thing imo either way. I have had dogs that didn't like at all, but they weren't necessarily trained with positive reinforcement only.
My CorgiJack licks until your hands, feet, whatever are completely saturated. And he's very intent, almost crazed when he does it. Could it be just an expression of nervous energy?
Orion gets me up in the morning by licking every inch of my face. If I complain and try to cover my face with sheets he gets more determined to give me more kisses. Also if we lay on the floor we are forced to get kisses. Laika on the other hand loves licking toes and legs, but sometimes she starts biting and gets in trouble, haha