My approach is similar to feline fanatic's. I typically do not prewash my fabrics. For the first wash I use Synthrapol and ***lots*** of hot water. I have a front-loader at home (these do not use enough water to adequately dilute dye bleeds), so I take the quilt to the local laundromat and choose a very large commercial front-loader to be sure that there is enough water to dilute any bleeds. Synthrapol suspends dye bleeds in the water so they don't have a chance to settle in other fabrics (but ***only*** if there is enough water to adequately dilute the dye bleeds). Synthrapol requires hot water to do its job.
Another thing you can do is simply soak a stack of scraps in hot water in your sink *along with* a piece of the Kona fabric. Even if the fabric bleeds, if the Kona does not pick up the color you will be safe to continue. Bleeding dyes will bleed into hot water; you don't need any soap or lots of agitation to spot a bleeder.