Most of my woodworking has been with solid wood, and I almost always apply the same type and number of coats of finish to hidden and show surfaces to assure balanced moisture movement.
Now I am building a bookcase/cabinet built-in that will be painted. And I suddenly had to ask myself why I was planning to paint surfaces that will never be seen. Is there any reason to? e.g., the back side of the back panel or the underside of the cabinet floor. If humidity changes flow mainly through those surfaces and not the painted "show" surface, does it matter with plywood? This will be in an air conditioned space, and while it is on an outside wall our temperatures (in Atlanta) don't get as extreme as some areas.