Sorry that this comment is a bit long, but worth going into in detail I think ...
Posture is vital. But I think we have to keep a couple of things in mind about the history of the Lotus Position itself, its real benefits and purposes, monastery life, the Japanese tendency to fetishize the "correct" way (yarikata
) to do things, and the Buddha's and Dogen's central philosophical perspectives on Practice.
Yes, the Lotus Position has been the traditional yogic position for meditation for thousands of years, even before the time of the Buddha. And certainly the Buddha sat that way (as every statue of a sitting Buddha demonstrates). And certainly there are tremendous benefits to the posture in providing balance and stability conducive to 'dropping body and mind' and engaging in balanced, stable Zazen. In that posture, we literally can give no thought to the body. The comfort and balance of the body is directly connected, and conducive to, comfort and balance of mind.
But I would hesitate to go much further in attributing any special power or physical effect to the position itself.
First off, I believe the Buddha himself sat that way because, well, he needed to sit some way
for hours on end -- and the "lotus position" was then the custom in India for how people sat on the ground and very good for marathon sitting. It is a good way to sit on a rock or under a tree, which is what folks did back then (in fact, he may have sat with his posterior flat on the ground, by the way, or on a short pile of grass without a cushion or 'Zafu' ... which is very different from how we sit). As I said, it is very balanced and stable. But there is no evidence in the early Sutras and Shastras that he himself ever focused on the position itself as having some special power, always emphasizing the philosophical and psychological aspects of Buddhist philosophy far over the purely physical. Certainly, he did not encourage engaging in any other yoga positions as were common in India at the time (e.g., we do not stand on our heads as a normal part of practice), so I do not think he was a great proponent of the positional type of yoga itself.
When Buddhism spread to China, Japan and other countries, I believe that people continued to follow the custom. However, even then there have been a tremendous degree of small variations in the details of the Lotus Posture, e.g., hand position, back angle and such.
Now, when Zazen came to Dogen, well, it came to a fellow who also left us with detailed instructions about how to carry our towels in the washroom, clean our nose, bow, place incense, use a pillow while sleeping and wipe ourselves in the toilet (really, he did ... pages and pages on each). Dogen, like many Japanese of ancient and modern times, was something of a control freak who emphasized that there is "one right way" to do things (the aforementioned (yarikata
). I have seen Japanese get the same way about the proper way to wear socks and enter an elevator. Here is that wonderful short film that makes fun of it (I know that you have seen it 100 times):
and here is another
Now, that is not a bad thing, mind you, for Zen Practice. Don't get me wrong. It is the same mentality exactly as in "Oryoki" meal taking in a Zen monastery by which the simple act of eating requires dozens and dozens of set gestures that must be mastered in the body memory. It is conducive to many aspects of Practice, including focused mindfulness. Sitting in a set way such as the Lotus Posture has the same benefits of allowing the action itself to be forgotten as it is mastered by the body memory.
Also, of course, in a monastery ... like in army boot camp ... you don't want folks just running around and flopping down any which way they feel, eating and sleeping whenever they wish. Quite the contrary. Discipline is required, so naturally, is the demand that everyone march around the monastery and sit in exactly the same way.
If you look at Shobogenzo and other writings by Dogen, he actually spends very little time explaining the details of how to sit. In Fukanzazengi, for example, he explains the barebones act of sitting on a cushion, crossing the legs and such ... but for sentence after sentence after sentence he is focused on the "cosmic significance" of Zazen and the mental game. It is much the same when he describes how to carry a towel in the bath, wear our robes, bow or go to the toilet. He describes the procedure, but then is much more focused on the philosophical view of the act.
Bouncing a ball or changing a tire --is-- Zazen itself. Dogen was clear on that. Of course, you do not have monks changing tires or bouncing balls too much in daily monastery life, so Dogen did not talk about those. But he did talk about the equivalent for monastery life, namely, cooking food as the Tenzo, washing the floors, etc. Dogen was crystal clear that the Lotus Position is the whole universe, the whole universe and all the Buddhas and Ancestors are sitting in the Lotus Position when you and I so sit ... but he was also clear that EVERYTHING is the Lotus Position. It is clear that Dogen, too, loved the perfection of the Lotus Posture ... but there is very little talk, if any, in his writing about the power of the position itself (do not confuse statements about the philosophical power of the position with his asserting that some energy or effect arises from the position itself ... you will not find much of that).
In my view, Dogen's real message ... and the real message of Zen practice ... is not that there is only "one way" to do something in this vast universe. It is that "one thing" should be done with our whole heart-mind as the "one and only act in that one moment" in this vast universe. That is what Dogen was saying.
My teacher, Nishijima, considers the Lotus Position a pure action, one pure thing. He recommends everyone to sit in the Lotus Position if at all possible. I do too (too many westerners get lazy or scared and don't really try, or give it sufficient time). But these days, in Zazen, Westerners have begun sitting other ways such as in seiza or on chairs, all fine if balanced and comfortable such that the body can be let be. I believe that body-mind can be dropped away in those positions too if done with balance and stability.
An overly fetishized focus on the miracles of the Lotus Position itself is misplaced and misunderstands Dogen's intent.
Anyway, that is my position (pun intended). I won't budge.