When you got to clamp a frame and you want it to be square, don't bother using pipe clamps. The best clamp to use is a band clamp. These types of clamps are just a big nylon web with a ratchet mechanism on them. You wrap it around the outside of the frame and use the ratchet to tighten the clamp around the frame. Since the pressure is applied evenly on the frame, it's really easy to get a square frame as a result. Plus these things are cheap. For under $20 you can make sure all your frames are square.