You have just proved that there is a match for quilters no matter what material they use! I won't take a sheet as backing from a client as it is too difficult to get the tension as perfect as I expect my machine to give me.

Ahhhh, quilting, where you can use anything you want to make your masterpiece AND find someone else who will happily use the same thing!

And these are 100% cotton?!?

Any cotton material is suitable for you top piece. That was one of the things our ancestors used when they couldn't afford to purchase cloth. Depending on the project I've used sheets for material, as backing, and even as batting when whoever don't want a thick quilt.
I loved the 250 thread count. It was strong enough to hold the shape of the pieces I cut. The 250 count seems to be phased out in most stores as affordable sheets, and the 200 thread count is taking it's place.
All of my old cotton print material I've brought from stores were a lot thicher, maybe 300 thread count, only the plain standard material was 200 count. In January, before walmart brought back their material section, I went to Joanns for plain black fabric and it was so thin I could see through it, and the price had doubled. I had to use spray starch to firm it up so I could work with it. In comparision to a white sheet I brought for backing, I discovered they had a thred coult of 130.
In the end it is entirely up to you which material you want to use always considering what you want to use it for, and how you are going to finish it [machine or hand quilting].
p.s. Think, do you want to spend the time with a seam ripper taking out the hems?

200 ct should be fine! What you want to avoid is HIGH counts- the expensive ones- like the egyptian ones that feel like satin & cost an arm& a leg! LOL!!!

year ago I bought a bag of cotton/poly sheet remnants from a remnant shop in NC. I made a quilt out of them and it worked fine. The weight of the fabric was about the same so was easy to piece. You can do it out of 600 thread count if you want. Just have to respect the nature of the fabric and work with it.

The only thing I recall of using Sheets for backings, was that for handquilting it was harder to quilt. Otherwise I cannot see not using a quality sheet for backing if it suits the quilt you are making.. Especialy for kids quilts that will be washed often.

i looked everywhere for a white background with some little bit of pink on it...never found it, walking thru linen & things (i really miss that store) and there was a full set of sheets, white with small (1/4 inch) pink pocka dots...

perfect...worked great 6 yrs later still in one piece.