I have mixed fabrics on a lot of scrap quilts, especially one for a child that I knew was going to be dragged all over the house and the yard. Some of the fabrics look crinkled when you wash the quilt, and some look smooth, (depending on the fabric content, but that is the only thing I have noticed. I do wash all flannels or anything I think may shrink, but don't normally wash quilting cotton. I have even used t-shirt fabric in scrap quilts, just use the iron on interfacing before cutting the pieces.