Every quilt I have ever made has had a sheet as backing. I make my quilts for charities and I know they will get heavy use. I save a lot of $$$'s by buying the sheets at Thrift Stores. Those that have been used and laundered often are also soft.
I have used some sheets as fabric in scrap quilts. One is Paths to Piece that needs a solid and I chose a taupe sheet to cut and create the background for the scrappy paths. It made a very masculine quilt since most of my quilts go to males.
Being creative with fabrics also includes using your own judgment on what fabrics go well together. Thrift is also an "option" many of us choose to exercise in making quilts for charities. The less I spend on backing the more I have to spend on the fabrics for the tops and batting. This is my personal choice.
June