Licking does *not* always lead to biting, that totally depends on the horse. Just like you can hand feed treats to certian horses no prob, others will start to become aggressive about it. My current horse of the past eight years is fine with hand fed treats. She will also search for treats gently using her lips. She knows where the line is, and she knows she would get clobbered by mommy if teeth *ever* come into the equation.
Not all horses are like that, maybe by nature, maybe by handling. Licking is sometimes just that. Exploration of who someone is and what they have going on. I would also be much more wary of "oral activity" with younger horses. Because they are in a stage of testing limits, searching with lips and tongue will often lead to an experimental bite. I don't let younger horses put their mouths on me at all. It is actually my practice, for horses younger than, oh maybe four years, to not hand feed them. I put their treats in a bucket. After they get older, all of my horses have been able to accept hand given treats politely.
You never know, sometimes your horse is put in a position of being a good equine ambassador. When we went on vacation once, a family with small children came over to pet my mare. They asked if their little boy could feed her a treat. My mare waited patiently while I showed the little boy how to hold his hand flat and still, and then she gently lipped up the treat. He was thrilled (and of course wanted to do it again), but because I have taught my mare nice manners, a non horsey family had a positive experience with horses. We always need the backing of the public with horse issues, we should always try to make a good impression. I was so proud of my girl that day.