For the maximum potential for virtuosity, the guitar obviously must be in a certain place. If you hold it down at your knees with the neck aiming out at the audience, you can just forget any fast scales with the 4th finger on the bass strings. By contrast, if you sit or stand without the guitar, arms down, bend your left arm at the elbow and see where your hand comes up to (about 8 inches from your left shoulder), then put the guitar neck there, you'll see where the guitar needs to be to be played at the optimum level. It's all about access to the fretboard. If you want excellence, you have to start at the base of your pyramid of technique how you sit or stand and hold the guitar.
The next job is to keep the guitar stable. Look to the traditional classical guitar sitting position for a model of stability. The neck isn't going to wiggle when this position is employed. The guitar is supported against the chest, the inside right thigh, on the left leg, under the right arm, and even on the chair seat between your legs if you like.