I am not one who likes to prewash my fabrics - BUT - if you are not sure of your fabric content, I would prewash so they all shrink or don't shrink before you assemble the top. If you were making an heirloom or for some shows, you should be using 100% cotton. But, there is absolutely no problem with mixing whatever fabrics you have. Think about the old crazy quilts with cotton, denim, velvet, feed sacks, whatever. The important thing is that you like the fabric! It is your quilt and you get to make it whatever way you want!!! THERE ARE NO QUILT POLICE!