No easy answer. I think a lot depends and how you have cut the fabric in the first place. Did you cut it length of fabric or from selvedge? Are the strips perfectly straight on grain? When you press the seams, do you move the iron up and down, or back and forth? Do you starch your fabric? Each factor can cause waviness. So, I'd give it a try and see how your strips work. If you get waviness, I'd start starching the strips to fix it.