No particular need to finish either if the cabinet box is sturdy. But, lots of cabinet interiors are seen to some extent. The best choice for these is to used pre-finished plywood. It's a BIG work saver. You also want to avoid using oil based finishes on box interiors because the smell can linger. Shellac is quick and easy. Lacquer works if you are set up for it, and you can also use a waterborne clear acyrlic finish.
