I will toss in my two cents on this one.
First Pierre, Standing on flat ground balancing to gravity, should be the identical position that we end up in while sliding streight down hill, on skis. balancing to the foces that are created is what allows to happen. This is the basic balance position. If you believe that your position on skis is paralell to the trees while in a streight run, then you have a long way to go babe.
Ok about the foot. Try to picture this. THe foot works like a trypod simply put. More specificly it is like a "pentapod". Look at the heel as one leg and then four more legs that brace against the heel. yes supporting the heel is critical. Just as critical is the supporting the forefoot in an exact position inside the boot.
Any attempt at understanding the foot must incompass the entire foot not just one part. a comparison;think how much damage a bullet can create, but no damage will be done without the rifle.
While you are thinking about that, think about this. As anyone in the building trade would agree, an arch is a self supporting structure. Evedence of this is thousands of years old all around the world. Apply this to the foot.
There are actualy three different arches to a "normal healthy foot". One big one that goes from the ball of the foot to the heel. Another one running lateraly across the med heads, or from big toe to the little toe. another can be found opposite the large arch that goes from the ball of the little toe to the heel.
If you can picture this multiple arch structure you can see the relevance to only providing support for one end of two of the arches. It is not the most complete way to deal with the foot. Will it help? probably, but it does not deal with the whole story.
Hang on to your tupe
If an arch is self supporting why does evryone try to fill the arch with footbeds. Why not support the ends of the arches correctly by placing the ends of the tripod in such a way that balance is the easiest to create and maintain? Simply filling the arch and wedging under the heel does not control pronation in the ankle completly.
Pronation is skeletal colapse.
Spination is a muscular responce to how much pronation occurs.
The act of pronation is always followed by supination. The constant strugle for balance is dynamic pronation and supination working together. What if there is so much pronation that constant muscular tension is a result inside the boot. Will skiing be helped or hindered?
Oh to make my coment on heel pressur. I was gettin into my soap box mode. Yes pressure on the heel is just as important as pressure on the ball of the foot.
What I try to encourage is this have the victem stand in the boots and raise their toes. not the balls of the toes just the toes. relax onto the tripod of the foot. not any one point any heavier than another.
When you push down on the ball of the foot it is using the muscle tissue that is meat to open the ankle joint. The result of this is backward movement. If you use the muscles that raise the toes you will find flexing the ankle much easier. and you will find the ability to relax on the tripod in the foot.
I guess this should be confusing enough