i've mixed them many many times- since often i prewash if a fabric may bleed- some are pre-washed- others i do not worry about bleeding i at times don't bother- most of my quilts contain both- no one ever knows one way or the other when they look at my quilts-
i do always wash my quilts when the binding is done-as the last step to completion....but i see no reason to worry about mixing washed/unwashed fabrics- unless one may bleed.