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It's difficult to say. The only way to know for sure is to keep track of your mileage through many tanks of each and try and draw a conclusion from that. Even that may not be enough, since there are so many variables that can also change (temperature, wind, air density, your driving habits, the health of your car).
However, my vote is that you aren't hurting your FE. Since all gasoline in the U.S. is now required to have 10% ethanol, and ethanol raises the octane of gas, the "regular" (I agree with you on the fuel grading being dumb) gas may be 91 octane anyway. Many stations now charge the same for "regular" and "premium" gas.
But as I said, the most definitive way, and even it isn't very definitive, to know is through testing.
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