We know squat as a move from an early age. We use this position many times a day as children. In the circle of bodybuilders and people engaged in strength sports, a popular squat is considered as the foundation for building strength and muscle mass both of the lower limbs and the whole body. It goes without saying that during this exercise we engage a lot of muscle fibers, so we include squats in the range of global exercises, i.e. those that involve over 30% of all body muscle fibers. A great exercise, however, you must remember that it is also difficult technical.
The answer is: The squat should be deep enough until we can do it technically correctly. Finally, we should strive for the full range of so-called squat ROM (full range of motion) traffic. And here are the proofs:
„Effect of Knee Position on Hip and Knee Torques During the Barbell Squat” .
In the above experiment, Group A was allowed to perform a squat with knees extending over the toes. In group B, this possibility was limited. 
What was observed?
The conclusion is simple. For both groups, the knee strain relief is small, while the hip joint load in a non-natural position has increased above 1000%! So if you want to have a problem with your knees and spine, do a partial squat with a straight calf.