Charlie,
First of all, I was careful to alternate the growth-rings of the boards that make up the top, so as to minimize any tendency to cupping; and the buttons that attach the top to the frame should also prevent it. I also applied two coats of polyurethane to the underside of the top, as well as four coats to the topside, so the top should be pretty much immune to variations in humidity. If it did turn out to be a problem, it would be easy enough to attach cleats to the underside of the top, outside the frame, to hold it flat.
David