Further to what others have mentioned:
- Many years ago I bought some beautiful but expensive wood framed windows, doors and moldings for my house. Part of my job was to apply several coats of finish to all the bare wood.
- An older professional carpenter who helped design, make and install them gave some good advice. He said to make sure and finish both sides and edges as doing only one side will cause uneven weathering over time.
- Moisture, humidity and dryness may cause successive swelling and shrinkage episodes to any unfinished portions. You may not perceive this until the damage is done.
He said to think of it like a car's paint and what would happen if you just painted either the top or bottom surface.