Polyester batting does not shrink at all. Cotton batting (including 80/20) and wool usually shrink anywhere from 1-5% depending on how the batting is made. Most shrink about 3%, which gives a nice crinkled effect. Antique quilts have that crinkled look because they were made with cotton or wool batting.
Only some types of cotton batting can be pre-washed to pre-shrink. A traditional batting such as Mountain Mist Blue Ribbon 100% cotton cannot be preshrunk because there is nothing in it to hold it together in the presence of water; that's why it needs to be quilted every 2". A cotton batting such as Warm and Natural that is needlepunched, as can certain types of bonded batting. However, I'm not sure why anyone would want to go to the trouble to pre-shrink a cotton batting. If you want a totally unshrunk look, just use polyester batting which does not shrink at all.
Those of us who do not prewash fabrics tend to test suspicious fabrics before using them. I drop a small piece into a glass of water and watch to see if dye bleeds into the water, then rub the damp fabric against white fabric to see if any dye transfers that way. All I am trying to do is eliminate really bad bleeders. My first wash of a quilt is always in *lots* of hot water with Synthrapol, which suspends unset dye particles so they can be rinsed away instead of settling into other fabrics. Even when fabrics bleed, a lot depends on whether or not another fabric will pick up the bleed; not all do.