Dream Wool batting is a wonderful batting that does not shrink---warm, but lightweight- i live in a pretty cold climate & love my quilt with the wool batting & flannel backing- it is so cozy & warm!
personally i can not imagine pre-washing a batt.
if you want your quilt warm but flat you could always use a layer of pre-washed double sided flannel in the center in place of batting...fleece is also very warm, does not shrink & comes in different (loft's)