Prewashing with Retayne is a great idea, but it's more complicated than just soaking a bunch of fabric. It takes very hot water and 20 minutes of agitation, either by hand or machine, for it to work properly. Here's the instructions.
http://www.prochemical.com/directions/Retayne.htm

How to make sure fabrics are colorfast:

I just came across a very good article written by a woman who dyes a lot of her own fabrics. She did some research and found the best way to prevent the dyes in ANY fabric from running is to let them sit in a lot of water for at least 12 hours. You can read her article here:

http://vickiwelsh.typepad.com/field_...d-fabrics.html

It depends a lot on the batiks. I don't normally prewash batiks, but I do test suspicious colors. First I rub a small piece against some white cloth to see if any color "crocks". Then I let a piece sit in a glass of water will to see if dye particles bleed into the water. Finally, I rub the damp batik against white fabric to see if any color transfers.

Most batiks I do not bother to test, and most of those I test do not bleed. A bleeder gets a Retayne treatment. (A few batiks require two Retayne treatments. If the dye isn't stable after 2 Retayne treatments, it doesn't go into a quilt.)

Another precaution I take is with the first washing of a quilt. I use Synthrapol and a *lot* of hot water for the first wash, along with several color catchers, so if there is any slight bleeding the excess dye goes out with the rinse water. I also am careful not to let damp fabrics sit next to each other; right after the washing machine, the quilt goes into the dryer.

Maybe it's the brands of batiks I buy, but I have not found them to bleed any more than other fabrics. I know that home-dyed batiks can bleed a lot, though.