A very good question indeed.
What I have learned during the teaching process:
What is really important about the posture:
The chair should be at such a height that elbows are about the same level with keys or a little bit lower.
The sitting bones are the biggest connecting point of the body and chair.
Backbone is in a balanced position so that muscules do not have to compensate the wrong support of backbone.
Also the pianist should not be too far from or too near to the keyboard.
There are the main things.
I have discovered that if fingers are put to work in a right way and finger technique is good, there are no problems with hand position. It all comes naturally and looks really right.
Of course, to achieve the right finger technique is quite a bit of work
The connection point between the pianist and the piano is a very important moment. And it determines a lot. I think that what ever too fixed position is not good.
The idea and musicality can transform into music through the pianist and instrument the best way, if human body is flexible and free.