Great question Met - because there is no clear answer. My 2 cents on this is that the most instructive position on this "issue" is finding a way to understand that both sides are "right" at the same time.
There's a concept floating around PSIA circles that is labeled "functional tension". If we allow the legs to turn more than the upper body through the end of the turn, then tension is created in the core. If we don't the upper body stays square to the skis through the turn. So, from one perspective we are using functional tension at all times in a turn except for the brief moments in time that we are square to the skis (i.e. approximately in the fall line). From another perspective, the level of tension is always changing. From an even weirder perspective, some people may need to focus on being "loose" to let the tension develop while others may need to "grr" to make it happen. My personal observation with functional tension is that it transfer some of the workload from the extremities to the core is overall a more efficient way to ski so that even though I use it all the time, I don't get tired.
Functional tension can be used in other joints. In your ankles and knees, functional tension can work like stiffness settings in shocks. Going over terrain variations with too much tension (stiffness) or too little tension can produce a rough or bouncy/swimmy ride. The goal here is to use just the right amount of tension to absorb the terrain and minimize the impact on the upper body, just like setting your shocks for the ride that you want.
That said, there are plenty of places in the body and times in the turn when muscle tension inhibits performance. If you hold your arm straight out in front of you with your elbow locked and hold the palm of your hand at a 90 degree angle to the arm (facing directly away from the body, you can feel tension in your arm if you try to rotate your hand from side to side (i.e. waving). If you try to wave while bending your elbow (i.e. lengthening and shortening your arm), the tension feeling goes away. I use this task in lessons to demonstrate why we need to bend our legs/ankles to allow the feet to turn underneath us. When we eliminate that kind of tension from our skiing our movements are stronger/more efficient.
The reason I love the term "functional" tension is that to me it describes tension that serves a useful purpose. This helps to focus the debate of good vs bad tension.