Missy came to me with her tail already docked. If it would have been my decision, I wouldn't have had it done. I disagree with it because it is unnecessary and only done for cosmetic purposes. If you plan on showing your Yorkie, then I guess you really don't have a choice. I think that I remember reading the tail is docked at 3-5 days of age.
Anyways, all just my humble opinion of course.