Mule and Donkey Hairstyles
There are NO rules at all about the way donkeys and mules must wear their manes or tails.  Don't let any judge tell you otherwise.  Neat is the only wording. 

Both types (donkeys and mules) may have their manes short and upright, long and upright, roached, hogged, trimmed, banded, or lying over. 

Tails may be natural, trimmed, banged, shaved, belled. 

A well-trimmed mane can enhance a neck and topline, while a poorly groomed one can destroy the overall presentation.

A good clip job on the tail is not only useful if the animal will be in harness (to prevent haris from being pinched and rolled in a crupper) but can set of a good hip and make it look wider. 
(left) This is from an army photo. This mule's tail is shaved part way down, then has2 bells and a long tassel.  Note how small and thin the bells are - this is typical as most mule's tails are rather thin.  If you are using this to hair a model mule tail, remember that thinner is more realistic than thicker. 
Another Army photo (see the cannon packed on this mule?  The britchin does not fit this mule.  The tassel on this tail is even thinner than the first.
This sorrel's tail is shaved only at the very top, about 2" down from the tailhead.  This is common on draft and saddle mules alike.
Here's an interesting quote about mule tails from the book Shavetails and Bell Sharps, The History of the US Army Mule by Emmett. M. Essin (available through the Hee Haw Book Service):

"Civilian packers also had devised a system that Crook's packers used to enable all concerned with an expedtion to spot new, untrained mules.  When the supplier or quartermaster presented a new pack mule or saddle mule (since saddle mules had the same conformation as packers) to the train, the patron ordered an unfortunate packer to roach the mule's mane and clip its tail leaving only a tassel. A "shavetail" was easily recognized and approached with caution, never from the rear.  By the time the tail feathers had grown out, it had become a "bell sharp", which meant it would take its place at the ringing of the bell.  The widely used descriptive terms "shavetail" and "bell sharps" were also given to army officers.  "
This mule has had it's tail shaved about half of the length (of the total tail), nearly to the end of the tailbone (you can see where the tailbone ends at the point where the hair drops from it).  This is done to enhance a hip, but is a bit too far for most people's tastes. It would depend on the conformation of the individual mule to make a decision to trim this far.
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Manes and trimming the mule for show!