I have heard of more people having issues with covered wood than not covered. If the wood is not 100% covered ( which you can't do because the frame is sitting on the wood) you can get trapped water. Then you will have wood that stays wet and will rot faster.
I'll take the possibility of warranty voiding and still protect the underside of my A122..The rubber coating will also protect the wood from salt damage and other chemical road hazards.. I am more concerned right now with the wire runs and unsealed holes in the bottom.. Wires on mine are so tight that any flexing will cause shorts..