To my understanding, the various physical postures for meditation and, indeed, prayer are related to harmonizing the energy flow throughout the body.

So, for example, the Christian method of praying by putting the palms of the hands together basically creates an energy loop through the joined hands which promotes a more relaxed mental state.

Regarding meditation with an upright spine etc., my own view is that it also prevents you falling asleep :)

I've tried meditating in bed at night a number of times and have always failed to stay awake. Sitting in a comfortable chair, however, keeps me awake without any problem.

From my various studies of the subject of meditation over the years, I have come to a very simple and straightforward approach...whatever is comfortable for you is right for you.

Sometimes that idea can lead to you finding something quite unusual and unexpected as your preferred posture...on occasion, I actually find it quite beneficial to meditate standing up in this stance...