Well, it depends. I personally don't recommend it. As long as you only use low throttle, aren't towing or going up hills, and are in a cool environment you can get away with it.

But under any reasonable load one of two things will happen. Either your anti-knock sensors will work and save the engine, but your timing will be so far retarded that fuel economy will suffer, eliminating any money savings, or they won't work fast enough and detonation will severely lower the life of the engine, also eliminating any savings.

Just inflating your tires properly can save you more money than buying lower octane fuel.