I have used sheets on my charity quilts. They're sturdier than using muslin, so they will take more abuse, and the financial savings helps me to make more charity quilts. I get a lot of the sheets from 2nd hand and Charity stores, and I do like them just a bit worn, or at least washed several times, as they are softer than brand new quilts. But then they can't be too worn either! It's just a preference. And it's your quilt, so use whatever you want for the backing. The quilt police won't come after you, I promise!