6 months is recommended because at that time the grown-up teeth should be in and the remaining puppy teeth can be pulled by the vet while he's under. Our's was just(a few weeks ago) neutered he was just under 8 months. The vet said it was a bit late only because he had double teeth and the adult teeth were pushed a little out of postion.
BTW: I read around six months in "The Yorkshire Terrier", book by Deborah Wood.
Good luck!
