Funny, my gut tells me that the most pressure would be at the tips due to the deflection/decambering. However, the analytical side of me says that (assuming a constant radius/curvature of decambering along the long axis of the board), the greatest force would be at B and C. Reason 1, the thickness of the board being greater at B and C would mandate greater force to maintain
constant curvature with the thinner sections, A and D. Reason 2, A and D are essentially the tips of a lever so the force transmitted thru your feet to the edge of the board will decrease in proportion to the distance from your feet.
I would think one of the board builders would be able to shed some light on this one.
All that being said, I'm guessing A and C would be most important because A initiates the carve and C handles the weight shift when I change edges, but that's just a guess because I'm bad when it comes to keeping my board tuned. In fact, I doubt I could cut a piece of perforated toilet paper with my edges.
What I really like is the tennis raquet analogy. I think quadrant B is without a doubt the most important one to have strung... well, C if you're left handed or French. Duh!