We talking old school stuff here? you don't even have a choice with the new 6 speed sticks or with the auto 6 speeds. CAn't speak to the new 8 speed autos.
Well, Ford has always said not to tow in OD. My 04 Powerstroke said so right in the owner's manual. It's kinda common sense really.....at least to me. Towing in OD means on the slightest grade, you're shiftin to 3rd. Back and forth back and forth. "I" would simply slow down and use third. Call it my personal opinion if you want.