Taping the back down is a great idea. Always start pinning from the middle, smoothing the top as you go. With the back taped securely you should not have to worry about the back slipping. Pin, pin, pin about every 5-6 inches. Sounds like a lot but very worth it. Since I only hand quilt with a hoop it is easy for me to keep the back smooth as I go. Good luck, and happy quilting.