[Hakulyte]

Ideally, you'd want to sit as straight as possible. Not for optimal scoring, but so you don't crack your back and be semi stuck in that slouch position by default one random day that you decided to push yourself.

Posture is pretty important for this game, especially on long term. People usually just don't realize it until it's too late.

Bad hand posture can also prevent people from playing at their real best.

Elbows shouldn't support your hands. You will eventually crack them in a weird way after a bunch of years.
Shoulders is another thing that can hurt if you tense up too much.