I'm in the "don't prewash" camp, and I've had good luck with the following method twice, but don't know if it's quilty sanctioned or not. I had a purple batik sewn in with whites and light pinks and after testing a piece of the purple, it ran REALLY badly. I have a big deep sink that I filled with cold water and a quarter cup of salt. Stuck the quilt in and hand agitated it for a few minutes. Once the water turned pink, I drained and refilled the water. Repeat agitation. I repeated the process several times until the water ran clear during agitation. After that, straight into the washer with cold water on gentle cycle. I had no runs, the whites and pinks were really nice. I did this method on a green and white quilt with the same results. Let me say too, that once you start down this road, you can't stop until that water runs clear, so make sure you're not in the middle of something else.
I've heard Synthrapol (sp?) works very well on already pieced quilts. I have never used it, but would definitely give it a try!