The only recent time I've flown rental, the Hobbs was used, as the tach did not have an hour meter. They charged you for the Hobbs time, less 0.3 hours, as they knew that that would the proper amount of warm up time. It was a Schwiezer helicopter, so it did not have brakes.
To be fair from a fuel burn and maintenance perspective, the significant "cost" to operate the plane starts when you apply takeoff power, and ends when you taxi off the runway. Charging for time other than that is being greedy.