I don't prewash fabric, but do always wash a quilt in Synthrapol for the first time (and for subsequent washings if there is any color in the water). Synthrapol suspends unset dye particles in water so they are rinsed away.
You can "test" fabrics in a jelly roll by getting inexpensive clear plastic cups, filling them with water, and putting a piece of each fabric in a cup. Let sit to see if any color bleeds into the water. Also, afterwards, rub each fabric against a white fabric to see if there is any color transfer. If the fabrics pass these two tests, you can be confident that the quilt will wash up without bleeding -- especially if you use Synthrapol.