One of my first experiences was lying on my stomach, it was completely unexpected, because i thought it was only possible from lying on your back. I woke up into vibrations and it felt extremely easy to float up and out of my body.
Nowadays i spontaneously project from any position, but normally my side which is my natural sleeping position.
i can project my stomach very easy when i am sick or hang over...
Never wanted to do it this way, never use to it.
When all conditions are meet for a projection, I lay on my stomach. I like to do it, because no serious relaxation is required just a little attention on my neck.
I've found that if something is a waste of time then relaxation definately is. Relaxing the neck is enough for me, because its relaxation will spread out the entire body.
I've projected from a position where I am lying down on my stomach a couple of times and there are a couple of things I like about it.
One of them is that the tendancy to want to lift up the body when exiting is extirminated. when projecting while lying on my back, I run into problems when tyring to exit, because my head wants to move forward. Also, if I try and use the roll out technique, I tend to roll my body instead. But when I'm on my stomach, I tend not to have this problem. Of note is that I don't use a pillow when I'm on my stomach. I turn my head to the side and let my arms rest parrallel to my head. Of course, you should do what is most comfortable for you.
Another thing I like, is that for some reason it is easier for my mind to recreate the sensation of rising up out of my body when I'm lying on my stomach. I think the main benifit though is that my physical body doesn't have the freedom of motion to try and move up with the sensation. Whereas, if I was on my back, it is quite easy for my head to want try and come up for the ride.
Just some thoughts. Take care.