This is just my opinion, but I have found that authors and teachers instruct from their own personal perspective. We are not all wired the same way though and it is my belief that one needs to do whatever works best for their own particular encoding.
Most meditation teachers say not to lie down because you might fall asleep...some tell you to sit on a chair, some say sit on the floor, some say full lotus, some say cross legged is fine. A very few say do it however you are most comfortable and aligned. I find that one can meditate anywhere at any time. So, I do what I feel is appropriate at any particular point in time.
As this relates to Astral travel, I would say that one would be best served to do it however it seems appropriate at the time...Just my opinion.
Namaste
Mimi