I checked on FCI's website (
http://www.fci.be) for the yorkshire terrier standard!
TAIL : Customarily docked.
Docked : Medium length with plenty of hair, darker blue in colour than rest of body, especially at end of tail. Carried a little higher than level of back.
Undocked : Plenty of hair, darker blue in colour than rest of body, especially at end of tail. Carried a little higher than level of back. As straight as possible. Length to give a well balanced appearance.