Why not buy the really inexpensive ($1.19 yd) muslin to use for foundation piecing and then you don't have to worry about it. I read an article about recycling the dryer sheets for foundation piecing and the writer said that they had use the sheets several years ago for foundation piecing and the white strips ended up having a lot of yellow spots on them after the quilt was washed and dried. Also, a lot of people are allergic to the polyester fibers that come out of those sheets as they are washed and dried, especially children and older people. Also, in case of fire, anything with polyester fibers in it will melt instead of burn.