When we rode mules to the bottom of the Grand Canyon in 1997, britchin was used on every saddle.  What surprised me though, was that they didn't use breast collars for the trip back up the next day.  As there is nearly a mile difference in altitude from the South Rim (where we started the ride), to the Phantom Ranch near the Colorado River (where we had supper and stayed the night), the trails are very steep. I asked why they didn't use breast collars.  The wranglers stated that a good tight cinch was all that seemed to be necessary.  In fact, for the steep ride back out, going up, they didn't use the britchin.  It was just tied up out of the way to the back saddle strings.  Their thought process was that extra straps just gave more opportunity for saddle sores to develop on the mules.