I did a signature quilt as wedding gift. I use Kona Cotton - prewashed and they used pigma or ultra fine sharpies. I actually taped each square around the edges with blue painter's tape, so they wouldn't get into the seam allowance. If they are signing them there, I would either be sure there are pieces of fine sandpaper to put the fabric on to keep it from moving when they write, or do what I did, and fuse a piece of freezer paper to the back that will serve the same purpose. It takes a bit more preperation time, but at least everyone will have a square that doesn't move.