Right posture is actually quite important, and Shi Fu says that
it WILL affect the growth of wisdom. According to him, in terms of
liberation it's ok, you still can be liberated as an arhant. But to
develope wisdom as a Bodhisattva you should learn to get a straight
back. To him it is actually quite important that we learn to keep a
right posture. He also gave examples of famous masters like Zen
Master Hsu Yun (120 years old!) and his own master, Zen Master Dong
Chu, who grew very old and yet their posture is almost
perfect!
According to what I learnt from Thusness, posture also affects
one's mind stability. Mudra (hand gesture) is also another
factor.
Sitting straight is actually quite enjoyable. You just have to get
used to it, it takes some time. And as I mentioned there are long
term benefits: you won't get hunchback so easily when you grow
old.
You also won't fall asleep easily with right posture.
