As mentioned, the current ECU's can handle the lower grade fuels at a performance cost. I have noticed the difference in my 98 E320, however, the cost difference between the fuels is minimal.
I would recommend staying with the premium fuel and look for other ways to increase your milage like keeping your tires at the recomended pressures (I would say the most important and often overlooked) , lower a bit your cruising speed, avoid hard acceleration, and keeping the engine well tuned.