I run 87 (yes 87) octane in the Winter, and 89 in the Summer. I have never once had a noticeable knock under any acceleration, load or RPM combination.
Note however that in California most of the gas stations have a 10 - 15 percent ethanol blend, which has an effective octane a few points higher than the rated octane. That may have something to do with it.
My 1200RT by the way, had a 91 octane recommendation but was specified to safely be able to run 87. It also had a knock sensor and timing retardation system. If I ran 91, no knocking. If I ran 89, no knocking. If I ran 87 and wound the engine up at max RPM at wide open throttle, then "RATATATATATATATAT!!!!!!" for a few seconds until the timing retarded. Sounded like a wire wrapped around the wheel was whacking on the engine. I ran 89, as 87 caused enough of a hit in mileage to more than offset the cost. No difference between 89 and 91.
Bottom line is you will hear detonation before the point where it will cause damage.