"When the neck and head are upright, in line with the *spine*, the path of the breath (prana) is clear, inhalation and exhalation are smooth and *unrestricted*". MMY Gita VIv13.
The importance here is to keep the channel of the sushumna clear,(1/100th the width of a hair) ideally that would be the posture the Guru Dev himself meditated in which would be padma-asana or the Lotus posture. Most of the great Yogis are seated in this position, some day we too will most likely adopt this posture to enable us to function effortlessly from, "the home of all the laws of nature", like Guru Dev was. What we need is 8000 Guru Devs doing the Siddhis in a group, *really* functioning from Aham Brahmasmi, now that's real power not this nonsense we see today, it's a good start, (like everything with the tmorg) but that is about it! TM is 'Yoga-lite' for modernity, hence any comfortable sitting position will suffice, at least initially........