Well all I can tell you is that I sleep with one under my head and one under my knees. I thought it was a little crazy at first but instead of having my knees locked out all night it feels so much better in the morning when I get up. See, I can't sleep on my left shoulder too much 'cause I hurt it in my senior year on the mat. I can't sleep very long on my right side because I popped out my first 5 ribs from my sternum during a JJ match. Therefore sleeping on my back or stomach is the most comfortable. My right knee bothers me as it is and with a pillow under them keeping it bent helps it not get stiff. Hopefully that answers your question.
I sleep with about 3, very comfortable, pillows. On a soft foam mattress that lies on top of a regular boxspring mattress. Very good nights sleeping.
Last week I spent a week in a university dorm, on a creaky, hard, boxspring mattress. With one, thin, pillow. And an abrasive quilt that looked liked it had seen too many years.
Must've been a relatively good boxspring regardless then. An old mattress that has springs jamming into from various angles and/or is sagging produces a very poor night's sleep in my experience.
I use two pillows, but that's mainly because I tend to wake up with a sinus headache if I sleep with just one pillow. I don't think it's necessarially better for my neck.
I think its just a matter of keeping your vertabrae relatively lined up (schientifically schpeaking, of course!) When I'm on my stomach or back I tend to use my ultra-veteran mooshed out flattened pillow, but if I'm on my side, I find its a little better to roll a little closer to my fiancee and utilize her slightly puffier pillow!