By coincidence, I spoke to some Ford engineers yesterday on this matter. By using premium fuel in todays vehicles, the manufacturer can get the best performance and emissions possible. Primarily, it's fuel mileage they are concerned with. The auto companies need to get the most mpg out of every vehicle they manufacture. If it means putting a sticker that says "Premium Fuel Only", then it's a no-brainer.
Using a lower grade fuel will degrade performance slightly but whether it is econically better depends on the price you pay and the decrease in mpg. I would advise you use premium if you are towing though, since the engine will run cooler as the timing will be more advanced. All engine controllers that we design here have knock sensors in them so the engine won't ping regardless of the fuel you use.
I myself drove an ML320 1500 miles from Houston on Regular Unleaded this last weekend and averaged 23 mpg with nary a ping.
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)