No pillow?

I'm reading 8 Steps to a Pain-Free Back and have just begun trying stretchlying. I was wondering if I absolutely need to use a pillow, and whether the recommendations are the same for a person with a straight cervical spine. I used to sleep on my back with a thin foam contour pillow, but after x-rays recently showed that my cervical and upper thoracic spines are completely straight, I've stopped sleeping with a pillow at all, and am using a rolled up towels, about 2-3 inches thick, under my neck. From the book, this seems to be exactly the opposite of what I should be doing. But, the chapter on stretchlying doesn't specifically address overly straight necks, only overly curved ones. My doc thinks it could be from whiplash from a car accident some years ago. I'm doing PT on it, but the therapists haven't said anything else except that it was fine for me to use a towel. The other comfortable position at the moment is with no pillow at all, but again the book doesn't recommend this. What's the best position for a person trying to remedy an overly straight neck? Am I doing myself more harm than good using a towel as a neck support instead of a pillow? Thanks.