If you play guitar for any length of time it makes sense to be comfortable.There are two seated positions the casual position and the classical position.
The casual position. This is the most used. John Lee Hooker shows the waist of the guitar over the right thigh. 
The neck of the guitar makes an angle to the body.
The classical position. The foot stool which is a defalut with classical gives an uneven posture. The gluteals are engaged differently on the left and right and there is more imbalance on the spine.
