Sleep position is actually one of the current experiments on Stephen LaBerge's Lucidy Institute website. I haven't done it myself as I find it very difficult to fall asleep on my back. There might be something to it though, I mean they say lying on your back makes it easier to enter SP. Like others have said though, if you already have preconceived ideas of how positions affect you then it's probably largely placebo. If you can use placebo to your advantage though then that's always a good thing!