I don't pre-wash primarily because I'm lazy.
I've got to starch and press the fabric before I use it anyway, so I might as well get it wet enough to shrink it before I press it.
But I also wash my quilts as soon as they're done, so if I've pre-washed the fabrics before cutting and piecing, too, then the fabric is even less "new" looking. I like my quilts to be puckery but I like the fabric to be as new looking as possible when I give the quilt to someone. :)