you need to wait until your quilt is quilted & bound before washing- otherwise you will have quite a mess to deal with. don't worry about it---unless you have deep, saturated colors that may run- if that is the case then when the quilt is completed toss in color catchers- or use synthropol (an agent that will keep running colors suspended in the water-not on other fabrics) as for the shrinkage---the quilting and batting will play a roll too- I've backed many quilts with minkie- sometimes I pre-wash , sometimes I don't, often my quilts contain both pre-washed and new (unwashed) fabrics- I've never had a problem because of that. since the minkie is polyester you will want to wash your FINISHED quilt in cool water- I toss mine into a regular dryer cycle- they turn out just fine. my concerns with prewashing are usually based on- is the fabric a loose weave- needs to be tightened up? is it a thin flannel? will it bleed? (I test to see) does it feel funky? does it smell funky? does it have marks, soil, stains on it-sometimes from spending a long time on the bolt in a shop?