I have not done the bed of my truck, but I do have some experience in woodworking and hardwood floor installation.
If you are going to have any wood that is exposed to the elements you have to urethane every side of the board. This will help to prevent the effects that humidity has on the entire board, not just the top. It is the same concept as protecting the underside of sheet metal to prevent rust.
I have had luck with the Minwax Helmsman Spar Urethane, I use it on outdoor furniture and high wear wood pieces (bar tops). I have not had any major problems with expansion or contraction of the outdoor wood pieces.
