2008 Other Mercedes Benz Models Fuel Octane 87 vs 91

I have a brand new Mercedes ML550 and I was wondering, with the Skyrocketing of gas prices, and the stations gouging the 91 octane (premium). The prices are not 20 cents higher, like they used to be, its more like between 30-50 cents more expensive than regular unleaded (87 octane) per gallon. My car maker suggests premium, but I have seen my car on a list at an ethanol station. Is it really necessary to put premuim (91) in my vehicle, or can i just use regular (87), or should I go for upgraded (89 octane) to reduce costs, or does it even matter if you know you will not even keep the car after the manufacturers warranty expires?