I don't wash quilts before entering them into a show. Instead, if they need to have markings removed I use a spray bottle of water since I only use water soluble markers, soap slivers, or chalk depending on lightness or darkness of the fabric. I block all my quilts before sending them out by pinning up on my design wall then spraying with water until wet and re- positioning the quilt and re-pinning as needed to get it completely flat and square.
I also go over both top and back with one of those lint rollers before sending any quilts out to shows.
Rob