Ask 100 woodworkers the finish both sides question and you will get at least 105 different answers.

Bob Flexner, a finishing expert for several magazines says it's not necessary, and lists his reasons as well as when you might want to. The fact many old furniture pieces were not finished on the bottom (tables) or insides (dressers and chests) and did not warp/crack/etc also lends support to this view, as well as they were designed to correctly handle any wood movement that may come up. Also, depending on where you are located, it the table is going to live in an environment where the temperature and more importantly humidity are controlled (think an air conditioned house, or the South West United States), you are not going to have the humidity swings that could cause problems.

However, another reference mentioned the American Institute of Architects (AIA) sets the standard for commercial woodworking. The standard requires that the finish be balanced whatever is done to one side must be done to the remaining 5. This prevents the wood from moving in unequal ways. I could not find this documented on their site.