I used muslin for the first couple I did (I have the Grace Z44 for hand-quilting) ... not sure if my tension was too tight or what, but my muslin only lasted one project, two if I resteamed/realighed it. I switched to lightweight denim and am working on the same piece of denim now 12+ quilts later.
I serged both sides of it, then folded & sewed the seam. Lays beautifully flat, and holds up well. But I also found another trick that works better than the pins... as I rolled my quilt, the pins DID actually make a difference and to eliminate the bumps, and possibility of tearing my qood quilt fabric, I sewed with water soluble thread a 6" length of muslin top and bottom of the quilt top. Makes for easy quilting every time now, with no worries. :) Have fun! I love my Gracie