I don't prewash my batiks, for the most part. When I wash the quilt for the first time, I take it to a laundromat and use their largest front-loader (provides as much water as possible to dilute any dye bleeds). I also use Synthrapol as the soap so that any loose dye particles are suspended in the water rather than settling into other fabrics. I remove the quilt from the washing machine as soon as it is finished so that damp fabrics are not sitting next to each other for any length of time. This has worked for me.
Prewashing all the fabrics that I bring home would take too much of my energy. I prefer to save it for actually quilting.