Quite a few years ago, I received stretchy knit fabric already cut into triangles and squares from my great-aunt in Utah. I have been handpiecing the blocks and was afraid the stretch was going to be a real killer once it went on a bed. I found some cotton in a coordinating pattern, tiny floral, that I'm going to use to sash it to give it some body. I've never done it before so we'll see what happens. Since my aunt had taken the time to cut it all out, it has sentimental value because it was from her. I think you can use just about anything in a quilt. They are works of art, individual and beautiful to the creators. Good luck.
Wendy