Something as simple as arching the foot creates tension in the body, not only will this cause physical problems long term, but will have a negative impact on your ability to give 100% focus to creating beautiful sounds.
Loafing on the sofa with a guitar may seem relaxed, but much of your concentration will be on creating a stable position for the guitar and not on creating freedom of movement of the hands. Even the wrong height chair will affect your ability to play with ease.
I remember the first lesson with Craig Ogden… shoeless! He was very concerned that my toes were relaxed and not curling - a good point that we need to be relaxed when we play, but not very likely playing in front of such a giant of the guitar world. I remember the chair I had to sit on being very high and having to use his fattest book to elevate my left foot so that the Gitano rest didn’t slide down my thigh (I opted for Harry Potter rather than the complete works of Shakespeare for some reason).
This video looks at devices (footstool, Dynarette, Gitano) to help elevate the guitar to enable a position which gives hands easy access to all frets and stability whilst shifting.