I have always prewashed until I started making kaleidoscope quilts. They tell you to not prewash as the fabric sizing keeps the fabric more stable when you are lining up the layers. I also have made a couple of kits which had 2 1/2" strips or FQs which I didn't prewash. I do wash it when it is completed as my furry quilt supervisor does love to lay on everything while I am working on it. (she is just making sure that the "energy" is right!!) I use 100% cotton batting that shrinks about 3% so I always hope that the fabric shrinkage will look OK with the batting. So far, so good!