GCR 9.26.3 (9.3.27 in the updated GCR), second paragraph:
If the fuel filler cap is located directly on the fuel cell, a check valve is not required, provided the filler cap is a positive locking type and does not use an unchecked breather opening. If the filler cap is not located on the fuel cell, a check valve must be installed on the fuel cell to prevent fuel from escaping if the cap and filler neck are torn from the tank.
Most formula and sports racer caps are on the cell, but they usually do have an unchecked breather. If that description fits your car, you need the check valve.
Dave