This is something I found:
Teething: Yorkies and toy dogs in general have a tendency to retain their puppy teeth, especially the canines. At around 5 months of age you should start to watch the teeth closely, a perfect bite may go off in a matter of days. You may feel it's not important for a correct scissors bite, but this is the first step of the digestion process, proper chewing is important. The other importance is that the proper bite stays cleaner, over lapping misaligned teeth are a good source for bacteria and tartar to build rapidly.
http://www.yorkie.org/yorkie_puppies/puppies.html