Should you be letting your dog lick your face? The answer to that question is of course, up to you. I will say though, that if you were always one of those people that always dropped the "a dog's mouth is cleaner than a human's" line, you're about to be in for a big surprise. And no, I don't mean that as in you're going to be surprised about how right you were... More like how right you weren't.
Studies show that dogs and humans have roughly the same amount of germs and bacteria in their mouths. The question is, what kind of bacteria? Because they have different kinds, it’s hard to compare the two. Dogs' mouths have hundreds of species of bacteria with significant differences to a human mouth. Only 16% of their bacteria overlap with human bacteria.
Another study says that there are definitely harmful pathogens in a dog's mouth. A separate study in PlusOne in 2015 showed that oral-to-oral transfers of bacteria from dogs to owners can cause gingivitis. Also, porphyromonas gulae can cause inflamed gums and even tooth loss. This disease is very rare to begin with humans, but very common for dogs to carry; and studies show that 16% of owners have contracted it.
There are even some bacteria in dogs' mouths that are antibiotic-resistant, which means that if it spread to a human it would be very hard to treat. On top of all this, if you happen to have a cut in your mouth or bleeding gums, etc. your dog's bacteria can get into your blood stream and cause infections.
Also, while it is known that dogs have some small antibacterial properties in their saliva, it isn't good to let them lick your wounds. Case studies have shown that by another mouth licking your wound, you can contract something called Pasteurella. A Pasteurella infection can be treated with antibiotics, but is dangerous because it has been known to help promote other infections and diseases. So, long story short: no, you shouldn’t let your dog lick you in the mouth. Licking your face is okay, but I would definitely recommend washing up afterwards.