I suspect most airlines have a standard en route mach number to maximize fuel economy. Its like driving a car, you can go balls to the wall and operate at the maximum mach number, but you will lose fuel economy. Also ATC will slow you down when its time to decend into crowded airspace. Most airliners cruise around .78-.80 mach with a maximum operating speed probably around .85 mach. I would hope pilots have some leeway with the airline to shave a few minutes off the flight during delay situations.