While attending a Revit Structure user group in San Diego (last week), my first chance to visit finally, Behzad was demonstrating some of the new extensions, the Bridge Extensions content available via your Subscription account at Autodesk.
He showed something that, on the surface (pun intended), doesn't dawn on most users. The graded region tool isn't the only way to get your Net Cut/Fill results. This is true because Revit really only cares that one surface is existing and another is in a future phase. In other words you can model one surface in an earlier phase and model another one in a later phase and the properties of the newer surface will show the net/cut/fill values compared with the earlier surface.
One condition: New surface must not be created outside the boundary of the earlier surface. If it is...it won't compare them, no cut/fill result.