The basic principle is that balance of body is hand-in-glove with balance of mind, one nurturing the other, truly just body-mind. For millenia, the Full Lotus, and to a lessor extent, the Half Lotus have been considered positions of great poise and balance. The Burmese position is also very balanced. The lifting of the rump, straightening of the back with slight curvature of the lower back, the stability of the legs with good circulation, the comfortable head position ... all lend themselves to our forgetting about the body during Zazen. Once mastered, they are intended as incredibly comfortable and stable positions ... not torture
My teacher, Nishijima, is against Seiza and, even more so, chairs. They do not provide such balance in his view, and furthermore, were not the tradition in the Zen schools. Now, the official "Soto" school line (for Westerners, at least) is that chairs, Seiza and Burmese are acceptable ... if not ideal.
Nishijima makes the valid point that many Westerners give up on the Lotus postures for lack of trying, lack of giving it time and stretching. He is right. He may be a little stubborn in not yielding on this issue to people's needs who have legitimate physical issues, and I sometimes think so (this is a very Japanese attitude). But most westerners give up much too easily.
So, you should try many ways and make up your own mind. However, if you are physically capable of Lotus or Half Lotus (or Burmese), that is the best I think.
The philosophy around Treeleaf Sangha about sitting is that everyone should try out for themselves, and adjust, the fine points of sitting Lotus (Full and Half) and Burmese. Seiza and chair sitting, is tolerated as maybe necessary in some cases, if there is a true and uncorrectable physical need.
So, please tinker away with the Full/Half Lotus and Burmese in minor ways. You will know when you are balanced because, quite simply, you will feel balanced, and generally comfortable in sitting for long stretches day after day. The proof is in the pudding. Just make sure you are not sagging in the back, that the back is straight, that the chin is tucked in and the head not dropping forward, and that you are not leaning to the side.