Thats not true dnort. Below parallell is a much more natrual position to stop the squat, some research has actually shown that theres more force on the knees the higher up you stop. What hurts the knees is if you go ass to calfs and sit down there and rest with your hams against your calfs. This will cause the knee to want to "open" if you catch my drift? Because the calf and hams will press against eacother pressing the tibia and femur in different directions, then the tendons and ligaments in the knee will have to work to prevent that from happening. But aslong as you don't rest down there, it's not so much of a problem.