I don't know if the fuel grades are different in your area, but here we typically get 87, 89, and 93. Since Infiniti recommends 91, I could technically use half a tank of 89 and half a tank of 93. (Or 33% @ 87 + 66% @ 93).
The price differential between plus & premium, however, is only 10 cents/gallon, so that would only save me 5 cents/gallon, times 15 gallons/fillup = 75 cents times 26 tanks per year = $19.50 per year saved.
If the price differential between grades ever becomes greater, or if I find myself driving a LOT more, then it may become worth the extra effort. Until then, premium only.