Taking a chance here :), but I have to argue, to a point, the inside seat bone...as a caveat, I am using what works with my horse so take what I am about to say in that line. It won't be the first time I have or had different methods to get the same results :) My horse is green so we are playing the retraining from the track game with some interesting results and modifications to what I consider the "standard" way of cueing things.
As any OTTB, I had a problem getting the lead when tracking to the right..left was never a problem :) Trying to train for dressage, I started cueing with the dressage cue from the start, inside leg at the girth, outside leg slightly behind the girth, slight bend to the inside while holding the outside rein in contact, not pulling but just supporting, and seatbone shift also to the outside to lighten and lift the inside shoulder, though he isn't heavy on his shoulder now.
To make a very long story short, my horse decided, after a lot of experimentation and fast trotting, that he wants the inside leg forward of the girth and the outside leg at the girth for the canter cue. He needs to be bent slightly to the inside and I have to shift my weight slightly back and outside. He is balanced in the transition and down the straightaways but not completely balanced in a circle as yet. He has vastly improved over the starting efforts and I can get the canter, in the correct lead on either side, anywhere in the arena, immediately upon cue from either the walk or trot.
The basic cue is correct but as mine showed me, some horses prefer certain ways and won't read the "rule" book :)
Fair enough, and if it works, why not.
However, when you come to develop your dressage further, how do you then cue for canter half pass? Or canter a 10m circle, if you're sat to the outside, you're going to seriously impeed the balance of the horse by sitting on the outside seat bone.
As a green horse, I completely agree that if they get canter, just go, work on refining it once you've built a basic grounding in it, roundness, and correct lead etc comes later, but I still stand by what I say.. inside seat bone, inside leg on and impulsion, outside leg is there to keep the trunk in order, and as an extra for anything else you need, turning, cueing for halfpass, or anything else.