A wheel requires a true full contact surface for proper torqueing of the lug nuts. An uneven mounting surface will result in improper lug torque, and can result in wheel loss. That's in addition to vibration and wobble problems. If you look at the typical wheel, particularly steel wheels, the actual mating suface is not at the lug hole, but is on pads in between them. Use of washers puts the mating surface in a different area, and will cause the improper mounting issues. A full contact spacer is designed with this in mind, and provide a proper mounting surface for the wheel.
Guidelines concerning spacers is covered by the Federal DOT, and may not be listed in state codes.
