If you are not that familiar with woodworking, one of the facts about wood is that it moves around with changes in humidity, based on it's exposure to air. So if you seal only one side of the wood, the side that you didn't seal will start moving around over time, because it is still exposed to the elements. Therefore, the wood that is sealed on one side only will eventually cup and warp. This is why so many people's wood decks fail within 5 years or so. So yes, you do want to seal both sides of the wood equally, so that your wood will have less of a tendency to warp and cup. It will also prolong the life of the wood because a sealant is a barrier to rot. If there is a complete barrier to moisture all around the wood, the tendency toward rot is severly retarded. The more moisture that wood is exposed to, the shorter the life of the wood.
