I wash the finished quilt in a *large* washing machine that uses a lot of water (domestic front-loaders do not use enough water; laundromat front-loaders do) with Synthrapol. Synthrapol suspends unset dye particles in the water so they do not settle into other fabrics.
It's risky to wash or soak precuts because of potential shrinking. Once quilted, fabric shrinkage is not such a problem because the batting controls shrinkage. It's okay to prewash or presoak if you can deal with the potentially different shrinkage rates from prewashing or presoaking. This might require adjustments to seam allowances, for example, if the pattern requires exact finished dimensions. For something like a jelly roll quilt, variations in fabric shrinkage wouldn't matter.