Lying on your back is not necessary. I am a belly sleeper, but I do prefer laying on my back for projection and trance exercises.

Hypnagogic images mean you are quite deep and can probably go for a projection. I actually find it too deep, as it seems to compromise some lucidity when projecting from that state. Energy sensations generally occur before going that deep, though as you know that doesn't always happen.

Energy sensations are the key for me, and they can occur in any state, even when tossing and turning in bed, although this is rare.