Let me start by saying I'm not a boot fitter. I have had a number of boots custom fit, but mostly to accomodate my EEEEs (wide feet).
My opinion on boot fitting and canting is that the goal should be a flat ski with the body mechanics working comfortably and in such a way that the skier doesn't have to conciously adjust their stance for their individual alignment problems. Or to say it in another way: proper canting will let the person stand naturally on his skis and allow his skis to be flat on the snow.
Fitting a boot can adjust the body alignment significantly without ever adjust the angle of the boot sole with relation to the ski. Custom foot beds, grinding, filling and adjusting the boot shaft angles allow the fitter to have the foot, and subsequently the rest of the body, acheive a natural, comfortable alignment and permit the boot/binding interface to let the ski run flat. Sometimes what you can do all you need inside the boot so you have to work the outside of the boot.
Some canting is done to adjust body position over the skis to achieve a stance that is not necessarily natural as much as it is to overcome a tendancy to be too far back, too far forward, etc. This type of work would seem to be best addressed once a person is permitted to find his natural stance through proper fitting.
People should be careful not to rely on boot fitting, canting, detuning, retuning, new skis etc. to improve their skiing. Having well fit boots, a decent tune and good skis is important. So is technique. My experience indicates that many people with perfectly good equipment will blame their gear, the conditions, etc. before taking a look at their technique.
There are always two ways to solve a problem:
You can change reality or you can change the perception of reality.