Do I prewash? Yes. I wash all fabric that comes into the house with Synthrapol to help the batiks get rid of any excess dye. Does it always work? Nope. Sometimes it needs more than one soak. That's why I always check my fabrics before starting an important project like a show quilt. If you ever get in trouble Google search Margaret Solomon Gunn's blog post about bleeding. It does the trick. I've had to use it....
HOWEVER, I do not prewash my Island Batik Ambassador fabrics. Washing precuts can be a pain so I choose to leave all of it unwashed and wash later. When doing this I put Synthrapol in as well as color catchers and I haven't had a problem. I've been an ambassador for 4 years and no problems. Does that mean there never will be? Nope. That's why I always CHECK before using a fabric if it's a really, really important project.
I hope you get to quilt today!