I use sheets all the time. I buy flat sheets at thrift stores too. The vintage ones have a great feel and are usually a blend and wear well. I would say try one and see how you like sewing and quilting with it and how well you like it finished.

I love piecing the back of a quilt from multiple fabrics, inserting a strip of patchwork between different fabric pieces or using one or two blocks that didn't get used on the top. it is a chance to make a composition/ design that relates to the quilt top.

When we took our longarm training the trainer recommended not using sheets for backing because of possible high thread counts. So as a longarmer I prefer clients not use sheets as backing. If you are doing your own quilting remember there aren't really any quilt police. I use all cotton on my own quilts as I find polyester too hot. I'm going to try the diagonal backing method to save on fabric in my next quilt if I am not using wide backing.