I don't prewash because I like the added stiffness to the fabric making it easier to use and it stretches less, plus the added "bug retardant" that most mfgs put in their fabric to prevent bugs while storing to be sold. If I work on a project and then abandon it for a while I don't worry about bugs taking up residence in my work.
I have had only one quilt bleed when I washed it, and it was a brilliant red hue that bled. But I rewashed (BEFORE drying) with a little Synthropol and it washed out. I hand dye fabrics and Synthropol is designed to remove excess dye.
As for shrinkage I don't know as I never dry in the dryer, just outside on a line or layed on clean sheets on the grass.