There are many people who wait until 8 months to see if the dog retains any of their puppy teeth and then they have the neuter/spay done at the same time to avoid having to undergo anesthesia twice. Buster did retain his baby canines and had to have them removed. There is also a concern about having it done before the growth plates close here are some threads.
http://www.yorkietalk.com/forums/gen...-age-spay.html http://www.yorkietalk.com/forums/gen...ur-yorkie.html