I agree with Gaigai in that it is a matter of personal preference. I prewash and iron everything but I do it almost as soon as I get it. I have a dresser in my sewing room that contains my stash so no worries about fading.
I do it simply because I prefer the feel of prewashed fabric when working with it. The puckered heirloom look can be acheived with cotton batting or a cotton/poly blend like heirloom. It will shrink slightly after the quilt is completed and washed giving that puckered heirloom look.
FF