Jundo Cohen There are many different opinions on this. I am of the opinion that any stable, balanced, comfortable posture suited to your body (one size does not fit all) is appropriate. Basically, you know such a posture when you feel it, and are you own best judge (and it may not be only one posture for you). It need not be the Lotus, although the Lotus postures are marvelous for those who can manage them (I sit Full Lotus every few weeks just to keep in shape, but I think it rough on the knees). An excellent book I recommend with such a philosophy: The Posture of Meditation
A Practical Manual for Meditators of All Traditions
By Will Johnson
Reality is Dharma 