When finishing wood, you should finish both sides in the same manner and to the same degree. This prevents one side of the board from absorbing more moisture than the other absorbs. If you make anything out of wood, you should remember that what you are making is going to move. Even if you seal it tight with paint or varnish, it will expand and contract somewhat. But the less you seal it, the more it will move.
Some woods move a great deal with changes in humidity, others less so.
Then, too, how the wood was cut from the log will determine how much the wood moves. Planning for wood movement. This is the secret to success. If you plan for some movement, in the proper direction, then even though it doesn't work for you, it won't work against you. When a wide board is used and fastened along its grain, plan for the wood to be moving.