As has been said on this site many times before, it is your quilt you can do what ever you wish! In my experience for what it is worth, I have used poly/cotton bed sheets for my backing when I have used poly/cotton fabrics for the quilt top, no problems, I use the cheaper sheets with not such a hight thread count. I am sure what you have read is true if you go for the dense 'Egyptian' cotton sheets with a very high thread count. I am entering no competitions and I work mosly in scraps and recyled fabrics so using sheets is fine for me, it is down to what you can afford and what kind of end results you are after.
Gal