I feel it's totally acceptable to ask for half up front and a quarter of the balance before quilting. This way you're not on the hook for the batting, backing and quilting. If they fail to make the last payment at least you know it will be a finish and sell quilt. There will be no more waiting until it's ready to be stiffed. If you were hiring any house repairs they ask for progress payments to protect themselves. Cover the cost of materials and quilting costs that way it's your labor that you're out.