Personnaly, I won't allow my horse to canter uphill, but
prefer a gentle slow trot with the head on the ground or even a walk. Question
of energy sparing.
 
 From:
"A. Perez" <walkergirl@xxxxxxxxxx>
Subject: [RC]   trot vs canter
 
 
  For what it's worth, if
given his head while going up a steep 
hill, my gelding will almost always canter rather then walk, 
rack, pace or trot, so I assume that of all the gaits he can 
choose from, cantering is easiest, at least up hill.