I've never prewashed minkie fabric and have used it on lots of quilts, my queen sized bed quilt has a minkie back- it's been laundered many times- never an issue- or reason to pre-wash. as for the flannel- the flannel will shrink some- so I often (but not always) pre-wash flannels if I am mixing flannels and regular cottons- unless I'm making raggy quilts- then I do not pre-wash the flannels. quilting and choice of batting also plays a role in the amount of shrinkage you will experience in a quilt- really the only time I (always) pre-wash is if the fabric *flannel* is very thin-loose weave and/or the fabric I am using is a deep saturated color that may bleed.