Don’t tell the etiquette police, but I believe posture should be judged good/bad by the stress and imbalance it creates in the body. Case in point: sitting cross-legged. Lots of us have a favorite leg to cross, which means we probably have one side of the pelvis that hikes up and one side of the low back that’s tightened (namely the quadratus lumborum muscle aka the “hip hiker”). Our bodies want to maintain equilibrium so in response to the hiked pelvis, we’ll tilt our shoulders and upper spine the opposite way to make sure our eyes and ears stay level. So I guess sitting like a princess is both good etiquette and better for postural balance! ​