I would say that if all three layers are the same size, then you are probably safe. The x is generally to keep the batting (cut smaller) in place so it doesn't clump up. If your middle piece of flannel is not the same size as the top and bottom, you may want to go back and sew an x through the blocks. Raggyquilts are fun to make but can be costly so I don't think I'd skip the x step, myself.