Is it better for your back to stretch out to sleep without your pillow or not?

I heard if you want your spine to stretch and get straight you sleep without your pillow? I also read just put a pillow under your knees????

=To get a good night's sleep with a bad back, try lying on your back with a pillow under the knees to take the pressure off the lower back. Or sleep on your side with a body pillow that you can throw your upper arm as well as your upper leg over. If you're a stomach sleeper, try putting a pillow under your hips to stretch the back out a little.

Many people believe a firm bed provides more back support than a soft one. If your back gets really bad, try sleeping on a futon or pile of blankets on the floor.

Mario and the knee pillow