Fabrics shrink differently when washed on their own. Once they are in a quilt with moderate quilting, the quilting "ties" the fabric layers to the batting layer. This moderates any shrinkage. In other words, a fabric that might shrink quite a bit will not be able to shrink more than the batting allows. That is why mixing batik fabric with other quilting cottons is not an issue provided the quilt will be quilted. Differing shrinkage rates can be a problem in tied quilts and quilts that have the quilting lines far apart (say, the 10" apart max that Warm and Natural allows).
Bleeds are a different issue. I do not prewash fabrics, but to make sure I don't get any unwanted bleeds in my quilt I always do the first wash in the largest front-loader at the laundromat (the more water, the more any dye bleed becomes diluted) and use Synthrapol which helps keep any loose dye particles in the water so they don't settle in fabrics.