I just tried low grade and I can`t tell the difference. I didn`t hear any engine pinging or anything. I have slowed down my driving style anyway so I can`t tell if there is a power loss or not. What I do is buy 1 tank middle grade and then the next fill up is low so they are mixed, no problems here.
I run mid grade. But if I were on a long highway trip perhaps I would consider regular.
I don't think Toyota calls it a knock sensor but there is a sensor that picks up knock/ping that is most commonly caused by lower octane fuel & retards/advances ignition timming to compensate & eliminate it.