I would say that it is a good mental image to think about when talking about tipping the skis.
From a biomechanical basis, edging movements that feel like they come from the ankle actually come from the movement of the femur in the hip socket. In skiing when effecting edge angle there are 2 things to think about. One is the the angle of the knee joint, the other is the relationship of the boot, contact with the footbed/snow and closed chain movements. There was a video posted in another thread, by Razie, that talks about closed/open chain movements. Most movements in skiing would be considered closed chain movements. Basically closed chain refers to the foot being in contact with a surface and weighted. When in a closed chain, movements come from the femur not the subtalar joint. Even though your focus may be the ankle and that's where you feel it happening. The other thing to think about is that the leg cannot turn independent of the femur without the knee angle being 90 degrees or less. While the femur will become more disengaged with the leg as it approaches 90 degrees, it is not untill that angle that the lower leg will become completely disengaged. So all the talk of the mitre action of the subtalar joint is not applicable unitl then and in open chain situation.
I think the most important aspect we need to know however is that the movemnet happens in the hip, but more importantly the focus should be on the ski or edge itself during the actual attempt of edging. This comes back to the idea of internal versus external cues. While discussing the movemnet we most certainly need to speak of the movements and where they come from internally, but when we are working on the movement itself we need to think about external cues and how they determine the outcome. We as instructors/coaches neded to know the actual basis of the movements but that doesnt mean we need to make the students/athletes aware of that basis. It is more important that they figure out hopw to move in a way that gets the desired outcome on thier own.