I think I once read that using too high of octane actually is slightly harmful to the engine but I can't find the info offhand.
The reasoning was that too high of octane burns too slow in engines during moderate compression and doesn't reach maximum peak cylinder pressure. This would also cause some burning to continue after the exhaust valve was open. I suppose this can be seen under varying conditions that slow the burn in any engine with very short or no exhaust outlets (not normal).
Part of what I read stated that fuel economy could suffer because of the loss in efficiency. At the time, I even asked my wife to quit putting higher octane in her low compression daily driver since I felt it was a waste. She felt she was "treating"her car but it may have actually been slightly detrimental. I do know that I would rather err on the side of too much octane in the case of possible detonation.
