A knock sensor functions by DETECTING knocking. Therefore, detonation, or at least pre-ignition must occur before the sensor can tell the PCM to retard timing. Knocking and pinging will damage the engine over time, even if it isn't severe enough for catastrophic failure at the time.
Using below 91 octane should be viewed as a safety net, rather than a hammock. Use it when you have to, but not on a regular basis. Or engine life will be reduced, as well as catalytic convertor life.
