Right now there is a bit of a slight impedance mismatch between attempted reusability and the commercial launch offering of the full expendable Falcon 9 capability. On the plus side the dual role nature of the Falcon 9 v1.1 means they can take advantage of expendable launches with sufficient margin to test the reusability features, on the minus side it means that when there isn't that margin they don't get free test flights.

However, once the F9 v1.1 has been thoroughly tested in the reusable configuration they will stop offering the expendable option. Every flight will have the first stage flyback for reuse, and only payloads which allow that flight profile will be flown, leaving more than enough fuel margin to allow return and some contingency.

Larger payloads will simply be flown on the Falcon Heavy, which will reuse the lower stage cores from day one. Even if that means no cross-feed and a massive hit to the payload capacity the Falcon Heavy should still be able to loft payloads larger than the Delta IV Heavy and with reusability it should be able to do so at less than the current cost of the F9.