We had some friends to visit recently. Oscar (nearly 5 months old) sat on my lap and growled for half an hour.
I gave my mate some pieces of cheese and Oscar took them, but very warily.
I'm sure he wouldn't hurt anybody, and I like the fact that he doesn't make a big fuss of visitors.
If people visit and ignore him, he's much more outgoing, but how many people can ignore a Yorkie pup?
Also, if people he doesn't know stay for a couple of hours, he seems to relax, and he'll end up taking them toys to play.
When we're out, he growls at just about anything that moves, but he is getting better with other dogs. He's particularly fond of a little westie girl at his obediance class