I guess it depends on whether the ECU tries to advance timing as far as possible, even with premium, and then backs off as it detects knocking. I would assume that with proper premium, the ECU would be at its "normal" settings (full advance) and expect zero knock. So while it might pull back timing for lower grade fuel, it does so in a reactionary manner, and would keep trying to have it as far advanced as possible - that means there will be some knock. I don't know if minor knocking is detrimental, my gut feel is that it would lead to additional wear and tear due to uncontrolled pressure waves in the cylinder chamber. For lack of better knowledge, I stick to premium if it's available.