My favorite position: the left elbow propped on the pub table, a full pint glass of Diet Coke at the right.
OK, seriously ... actually, an open windway is a good thing, and sitting up with a flat back, arms relaxed, and both feet flat on the floor really IS the best way to go. But it's funny ... I played with lovely posture (at least in concerts or rehearsals or whatever), for what, close to 35 years? And in fact, when I pick up a Boehm flute, I still do. But then I play one of these things and I go all over the place -- I sit curled in on myself, I cross my legs (a big no-no in my flute playing day!), I put my arm over the back of a chair, I put one or both elbows on the table, I put the flute on my shoulder, you name it.
FWIW, I don't think it makes me a better flute player, and I do have a sore neck or shoulder after long sessions sometimes now. Though that could just be being 43! But anyway, I personally think the Boehm flute is much better balanced or ergonomic or something, at least for female hands, because all that junk doesn't happen when I play it. But when I play the simple system? Ugh, my college flute teacher would DIE!
Deja Fu: The sense that somewhere, somehow, you've been kicked in the head exactly like this before.