An article entitled 'proper posture required' put me in a tizz and out of kilter with my zazen for some time a few years back. The premise being that if you're not sitting in full lotus or atleast half lostus as stated in the fukanzazengi then you're not sitting zazen.
Being me I beat myself up about it, tried to stretch, tried to atleast get to burmese posture, ached limped, tried different height zafus. Went back to sitting on my bench for a while. Then back to the the pretzel twisting, reading articles on the web about the right stretches to do. Round and round and round again. Even when told not to worry, I did.
It was and never is going to happen, apart from anything else I have sports injuries that affect my knees and hips.
It still resides somewhere in me I'm sure, but the illness I've had , CFS/ME, has been a good teacher, as well as the other great teacher's I've had the fortune to meet and talk with. The proper posture I require is the posture that allows me to sit zazen. And that's I think, the proper posture everyone requires, the one that works for them.
Sometimes I lie down if I'm struggling as Alexander technique refers to as semi-supine, or the active rest position. Other times I sit on my bench. And that's been a quest in itself. Over the years my bench has changed in height and angle until it's just right for me, I've even added a cushion to it. What a revelation that was! My legs don't get so numb now and my upper back doesn't get the pain and tension it did. I've found a padded 'sausage' cushion is just the thing to support my hands and forearms to relieve the upper back pain.
I've also found that allowing my body to release and lengthen up, widen and expand has helped tremendously as detailed in this book on Alexander technique and this one too.
I'm sure plenty will disagree with me, but I seem to have found my 'proper posture' for now. But as we are a constantly changing living organism I expect that it will change, both as I gain better use of myself, but also as I re-act with changing circumstances.