I wash everything as soon as it comes in the door. I don't want any surprises later on, and I want to start out with clean fabric. Since everything in my stash has been washed, it's ready to be used. Sometimes that good quilt store fabric does bleed, or shrink unevenly, or otherwise behave badly. I want to know what the fabric feels like after it's washed, because that's what it will be like when the quilt's finished. (Some fabrics only have that shiny feel because they're loaded with sizing, and I want to be aware of that before I use it.) I want to be sure there are no allergens on the fabric, and that I've washed away any chemicals that may remain from manufacturing.
Quilts will still crinkle up the first time they're washed, even if the fabric was pre-washed. And good fabric retains the silky feeling after washing, so that really isn't a good excuse not to wash.