If you are going to do WILD get in the position that you fall asleep in or find yourself to most comfortable, laying on the back is merely a suggestion and is not necessary. As for having LD's and such and saying that you have better LD's on your back this is a myth at best.

I always sleep on my side or stomach and have LD's every single night so I think it is safe to say that the position in which you sleep has nothing to do with the quality of your LD's as it varies from person to person.