Much like Carol611 I never hold the same body positioning.  I have taken classes to learn how to hold my body properly.  The draw back I have found is that I end up concentrating on the way I hold my body instead of the actual service I am performing.  This is simply a hurdle any one could jump if they applied themselves.

      However I will say that without "Proper" body positioning what ever that my be. I have a great deal of clientelle, that is constantly growing.  And there is the debate of Haircutting being an art form and even though there are rules to art, some of the greatest haircutters in the world have grown to create there own standards and techniques from Robert Cremeans, to Vidal Sassoon, to John Masters.  

Same problem with my golf swing, so many things to remember, until it becomes habit.  Your absolutely right, that's why one on the reasons for my becoming a hairdresser is to use my own standards, refine those of others for myself finding the positions I can get into and am most comfortable with. If we define hairdressing as an art form, I would support the debate that we as hairdressers, stylist, etc., are artist definately, and like others we will practice our art.