Several factors come into play.
A 'comfortable' squat, with the back straight is important. Catcher's position needs to be balanced, so they can move to the ball that is off the plate, or pop up to make a throw. This will allow that balanced position for quickness of movement. Leg muscles should be 'pre-loaded' as much as is comfortable for the individual, meaning they should raise their rear end up a few inches, if leg strength allows and keep their weight on the balls of their feet, all again for balance and quickness. Later, beside or behind the throwing-side thigh will offer sufficient protection while keeping their hand closer to the mitt for quicker throwing and fielding of the low pitch.