I don't know about sleeping on your stomach, that seems to have been covered properly, but sleeping without a pillow is actually quite healthy. A lot of people don't use the pillow the right way and put it right under their head, instead of under their head and shoulders (which has about the same effect of sleeping without a pillow), and it ends up causing this sort of bend, so when they sit up straight, their head sits a little further forward than their spine. I'm not sure I'm describing it properly. Causes bad posture and makes it harder to sit up perfectly straight.
It depends how your spine is. I have quite a swayed back, so when I lie on my tummy I put a pillow or two under my tunny to help rise my back up otherwise it hurts. If your back hurts when you are on your tummy you could try this. If it doesn't hurt then I guess you have quite a straight back. Our spines are made to form to each new shape provided it isn't a crazy shape! Haha. It's fine to lie on your tummy as long as it's not causing you any discomfort.
I don't know if it is bad for your back to sleep without a pillow - it seems like it would be harder on your neck - but my chiropractor told me that you are more likely to have back problems if you sleep on your stomach.
It is if you have a stomach.
tummy*
