Disclaimer: I'm not a doctor and don't pretend to be a doctor; I'm just repeating what I remember of other people's warnings and my experience.
In kempo class, we were told to put our hands under our rears for all crunches, situps, 6-to-90s (legs out straight 6 inches off the floor travel smoothly to legs up (90 degrees off the floor) and back), and similar exercises. The extra elevation helped prevent back pain, and we were told it also prevented back damage. If you have poor circulation or are doing a lot of crunches, a thin pillow would probably be easier.
back pain from crunches
In kempo class, we were told to put our hands under our rears for all crunches, situps, 6-to-90s (legs out straight 6 inches off the floor travel smoothly to legs up (90 degrees off the floor) and back), and similar exercises. The extra elevation helped prevent back pain, and we were told it also prevented back damage. If you have poor circulation or are doing a lot of crunches, a thin pillow would probably be easier.