Slip plates are a must for consistent and accurate wheel weights. When you jack up a car in order to place it on scales, the suspension extends and unloads. You can roll the car back and forth or compress the suspension by bouncing on the car, but the typical tire has too much grip and will not fully relax. You would need 8 foot long roll offs in order to fully relax the suspension. This tire bind happens on the rear tires as well as the fronts. Although the rear suspension does not droop independently (with the exception of independent rear suspension vehicles), the read tire side walls flex as one side is jacked up at a time. The lower the air pressure, the bigger the problem. Asphalt circle track cars running significant amounts of static camber are especially vulneralble to this problem.
By using slip plates, the suspension is able to fully relax. Ride heights measure correctly and wheel weights are more accurate and consistent. We recommend use of some type of slip plate on all four tires during all set-up operations (scaling, alignment, etc.).