I’ve had the same concern.

Here’s my understanding:

If you’re “pushing your knee’s out”, hinging properly, using your hamstrings, and keeping everything tight, then your knee should be moving properly in either situation. Since your femur and tibia have some rotational capabilities (which can be seen clearly when using the popliteus to rotate the tibia) you can get the knee into a good position so long as you have proper mobility. Your ankle and hip then act to create stability and to connect and engage all of the appropriate muscles for the movement.

Pointing your toes outward is a compensation for tightness (or lack of mobility that most people have) that is usually felt in the hip area.

Pointing your toes forward increases the amount of torque generated within the hip joint – which increases stability and power. This can be seen in one of Kelly’s video’s where he takes Diane’s legs and tries to bring them together during a squat. He can’t do so when her toes are forward, but can what her toes are pointed out.