Shim Large Bows
Most of the time you can shim the cabinets as you go, but if there's an extreme bow in the wall (more than 3/8 in.), shim it out before you hang the cabinet. If you don't, you may accidentally pull the back off the cabinet while fastening it into place. Hold a level across the wall, and slide a shim up from the bottom (go in from the top when you're doing the top side) until it's snug. Then pin or tape it into place.