From a health perspective, it should be okay as long as your dog does not indulge in coprophagia (eating stools). Dog saliva has moderate antibacterial/antibiotic properties in it. Dogs in a natural state feed themselves through scavenging and hunting. The meat they scavenge is often ... not fresh. But they don't get sick like we do from meat that's turning, partly because they have this property in their saliva (also because of a much shorter digestive tract). In 1975, Heddle and Rowley found that secretory immunoglobin A (IgA) antibodies found in dog saliva had antibacteria properties, particularly against the bacterium Vibrio cholerae. In 1990, a team from the UC Davis reported limited antibacterial properties in dog saliva, against E. coli and Streptococcus canis. This is why they don't get violently ill when they exhibit coprophagia, whereas you and I would get very sick very fast. So although dogs have antibiotic saliva, it only protects them. If your dog has been licking or eating something yucky, it could get YOU sick, even though it won't get HIM sick. So if you do allow licking, make sure it's not around your eyes, mouth or nose. Just to be safe.
From a training perspective, it's okay as long as it's not dominant or obsessive. You may want to get help from a trainer to determine whether the licking is appropriate and if it's not, find ways to be affectionate with your dog without the licking. Keep in mind that if you permit your dog to lick you, he will lick guests, family members, and others who may be grossed out by it. Teach him a command that means, "stop licking," (such as "that's enough") and then distract him with a toy.
