As long as you are providing services as a professional you have the right to get paid. If you feel that you're not providing an extra service for the pre/post flight briefing don't charge for it. Personally I feel that even if I am not out at the airplane doing a preflight with my student, I am still providing a service by being at their disposal should anything come up in say a weather forecast or the pre-flight where they need my professional input. Even if we fly 1.5 hours on the Hobbs, I am still at their disposal for the full 2 hour block.