When there is load on the line(right behind the boat/2nd wake), shoulders square to that load is the most efficient way to resist as well as keep proper balance. If under load my shoulders are twisted slightly, I still believe one can hold all the load you need, but ones shoulder width has now narrowed in relation to the direction of the load, which means that the load is not evenly distributed throughout ones body. Therefore I am more likely to lose balance and have the ski fall behind me in the edge change, or worse yet shoot out from underneath me forcing my path straighter to the buoy. If you watch Nate out of his good side cut, he is completely open and never twists away until after he lets go of the handle, however, he is slightly twisted with his shoulders on his offside cuts. Not a big deal for him because his balance is so great, he is perfectly stacked, and his load is always in the right spot. Not to mention it is biomechanically much more difficult to do this on the offside cut. It's all about finding that balance in the edgechange, and it is much easier if there is less load, if you are stacked, and if you are as open as can be without giving up your direction.
*I think I need to do a better job of clarifying something else. Yes, shoulders square to the load when you are right behind the boat, but still keeping that downcourse position through the transition and edge change, or in other words shoulders would be facing downcourse through the edge change. I think this is where everyone fails with the whole open bit, it's not open to the pylon through the edge change, and it's not twisting away to the shore, it's shoulders facing downcourse so that you can maintain your direction*