Your hips should be as open as possible.
If you look at faster pitchers, their hips may not be open at all. So, I don't think this is true.
I have mentioned to you before, i think, that the rear knee turnover is the key - not the hip(s). It's the same story in hitting.
I also would have to challenge you to show me a high velocity pitcher whose hips aren't significantly opened by the time the stride foot has completely contacted the ground.
As to the hips vs. knee; here I think it may be a question of semantics. As I see it, turning the hips results in the back knee turning over. The hip drags the knee over. I'm not sure if you're saying the opposite or not; that turning the knee pushes the hips open. I see that as the same argument as "squishing the bug" because you would be teaching a symptom that can be accomplished without the correct mechanics. I think opening the hips and creating separation prior to upper body rotation is crucial to velocity and you can turn the knee over without opening the hips, but you can't pull the hips open without dragging the knee closed. I see the knee position as a checkpoint for correct hip mechanics rather than the engine for proper kinetic sequencing.