I let mine lick me only on the end of the nose if its in the face, but my girl has got me on the lips before by lunging at me. When she has, I would always wash it. It just seems gross to me and also seems like risky behavior.
Your Yorkie may not lick its butt (mine have never done that either), but they do sniff disgusting things on walks like bird crap sometimes, and I dont know for sure it they have contacted anything dangerous or not.
I also do give them rawhide strips at times to clean their teeth, and that is not exactly sterile. I sure as heck wouldn't get close if I gave them "Bully Sticks."
