
Interestingly, the KC breed standard says this:
Tail
Set on slightly lower than line of back. Must be merry in action and carried level, never cocked up. Customarily docked.
Docked: Never too short to hide, nor too long to interfere with, the incessant merry action when working.
Undocked: Slightly curved, of moderate length, proportionate to size of body giving an overall balanced appearance; ideally not reaching below the hock. Strong at the root and tapering to a fine tip;
well feathered in keeping with the coat. Lively in action, carried on a plane not higher than level of back and never so low as to indicate timidity.
Debs