The choice is entirely yours, but in my experience, wall pieces hang best with a stiff batting like Warm & Natural. It keeps them square and crisp looking and prevents them from getting droopy over time. Straight line stitching around the outside edge as well as around each white square should be enough if you decide to use W&N. If you go with flannel or nothing at all, be sure to use a hanging sleeve and not tabs or corners as your hanging mechanism. It will hang better with a sleeve to support it.