Highly recommended.
The use of these riser rings is required in any installation where water might naturally pool, or be splashed, around a vented fill in rainy or windy weather.
The ring raises the opening between the fuel cap and fill an additional 3/8" above the deck surface. This reduces the likelihood that water will enter the fuel system through the breathing vents under the lip of the cap.

The split version of the ring is designed for aftermarket use on existing installations. In most cases it allows the ring to be added without disconnecting fuel and vent hoses from the fill.
For new installations, rings are supplied without the split since they can then be installed before hoses are connected to the fill.
BR>NOTE: The ring will not prevent water from entering the system if the cap becomes submerged or substantially flooded. If this situation exists, the fill/vent must be reinstalled in a dry location, or replaced with a sealed cap and a separate vent located elsewhere.