Rick, I tend to think that since most of our driving is done on road, then our tires should be optimized for on road use. It's your decision, but you will lose mileage, and if you size the tire up you will effectively change your rear end ratio, giving you less pulling power.
I made the mistake of putting 265/75 r16 michelin tires on my duramax chev 2500. I lost 3 miles per gallon and the truck really didn't like the tires.
I talked to my Dealer and asked him to flash the computer in the truck to reset for the larger tires, I was told that they could have done it for a gas engine, but they cannot for diesels.
the tires were within the 10% range for re-flashing the computer for gas, but Chevrolet makes no allowances for resizing the tires for diesels.
I took the tires back to Sears, still on the truck and found out that I could return them if they were within 30 days of original puchase. I was inside the exchange window by one day.
Two hours later The truck was happy and so was I, New michelin XL's 245 75R16's all the way around.
Sorry for the long post just wanted to try to help