ive made many- some with pre-washed fabrics, some without; some with some of the fabrics pre-washed along with some that were not- all have turned out just fine. if you are concerned with bleeding, or chemicals in the fabric pre=wash, if those are not issues with the fabric you are using save yourself the step & wait till it's all done to wash it. pre-washing is often a personal choice- and there are those folks who state with great conviction that flannel should always be pre-washed 2 or 3 times before using it. I have made quite a few wonderfully beautiful quilts using flannels that were not prewashed. they have never become 'wonky'; shrunk unevenly; or had any other bad results. I tend to pre-wash if the fabric is thin, the weave loose- needs 'tightening up'; if there is a funky smell to the fabric or if there is a funky feel to the fabric- or if there may be an issue with colors running- otherwise I often don't worry about it. hundreds of quilts later- I will stick to wash when I think I need to- don't bother when it's not necessary; and don't worry about the 'quilt police' who think that's a HUGE crime.