For bottom mounting, nothing's wrong too wrong about routing bigger than the plate, except strength. Thinner is weaker, and the area over routed without the metal plate will be the weak point (area).
If you don't route too thin, though, it won't matter. Assuming MDF/MDO/partical board, leaving 5/8" is more than enough, 1/2" is probably fine, but 3/8" might not stand up to heavy play.