Miles per gallon and the minimum design octane are only parts of the equation.
Depending on the ECU and engine design, switching to 87 in a premium fuel engine at today's pump prices may not yield the best miles-per-dollar value.
In most markets, the price spread between regular and premium grades remains just 20 cents. When regular was $1, that was a large price difference. But at $2.50, it's under 10%. As the price of regular moves closer to $3 this summer, the price difference will dip below 7%.
Your ECU is going to compensate for the lower octane by retarding timing. Fuel efficiency will drop. If the mileage drop is more than 7-10%, then you should stick with premium fuel. You'll get more miles per dollar spent.
So monitor the MPG drop very carefully if you're switching to save money.
Of course, this doesn't even factor in the loss of fun factor with some engines that are optimized for premium fuel.