I don't prewash. I did some laundry experiments with clothing. I washed loads with and without color catchers. The color catchers generally come out pink. But the whites in the same load always come out white with or without a color catcher. So my red shirts always bleed. My white bras don't pick the colors up. The bleeding is NOT a problem.
Same way with quilt fabric. Some fabrics will always bleed. It's NOT a problem unless something else picks it up and even then unless you do something chemically to set that dye it will come out!!
As to shrinkage, since it's not the water that shrinks your fabrics, it's actually the dryer that causes shrinkage I figure my iron is way, way hotter than the dryer so that should take care of shrinkage.