I was just on equestrain life.com and there is a video in the english section about mounting. I did not watch it, but in the description is says "Make sure the mounting block is on the left side of the horse."
Isn't it better to get the horse used to things on both sides? I don't just brush one side of the horse, or hose one side. I even lead my horse on the left and right. I would also think that it would cause strain (even using a mounting block) to the horse's back. Wouldn't switching mounting on either side cut the strain and build that muscle on other sides? Other then tradition why must we mount up on the left?