I didn't find that it helped much in the old days, when I prewashed. Now I never prewash unless it's flannel; leaves me with a lot more time to actually quilt! I wash all my quilts in Synthrapol after they are made, before they are given away or used, and have never had a problem.
I do test fabrics that I suspect might bleed by dropping into a glass of water for awhile, then rubbing against white fabric. If the dye bleeds or crocks, I will wash it in Retayne and test again before using in a quilt.