I would not prewash because there are too many risks. Fabric shrinks differently when washed on its own compared to when it is first quilted to a batting and backing and then washed. Once quilted moderately, the batting takes over and prevents fabrics from shrinking outrageously; in other words, the batting stabilizes and moderates the shrinkage of the fabrics.
Risk is also added because you are dealing with both applique and embroidery. If the background fabric shrinks in a disproportionate way (old fabrics can shrink much more in one grainline direction than another, for example), the applique and embroidery may become distorted. If you wait until the background fabric has been quilted, the shapes and forms of the applique and embroidery will be preserved.
Some of the dyes may bleed when washed, but this can be controlled by the way you wash the finished quilt. The best way is to use Synthrapol (suspends unset dye particles in water so they don't settle into other fabrics) and ***lots*** of water to adequately dilute any dye bleeds. The easiest way to do this is in a top-loading washing machine. Fill with water, stop the machine to prevent machine agitation, hand agitate, advance the machine to spin to spin out the water, etc. Refill and re-spin as necessary until the water runs clear, then immediately lay out flat to dry (you do not want wet fabric to remain in contact with wet fabric).
Another advantage to quilting before washing is that the batting and backing layers add stability to what may be slightly fragile block fabrics.