i use whatever grade is recommended for the car. going above the recommended grade doesn't get you any better performance, and more than likely gives worse fuel economy (in most situations).
now, if you've modified your car, you may have to go to a higher grade, depending on what you've done, or if you are getting detonation (ping/knock), then a higher octane might take care of that. if your car is pinging with the recommended fuel grade, then you should get the car looked at. could be a symptom of a larger problem forming. or it might just be in need of a tune up.
but, unless you're car specifically calls for a higher octane, stick with the lower octanes, except to stop pinging.
the higher the octane, the harder it is for the fuel to combust. which is why forced-induction cars and high compression cars call for it. in those cars, when using lower octane fuel, the air-fuel mixture is actually combusting on the compression stroke, before the sparkplug fires...and that's what knock/pinging is. so a higher octane is used. because it's harder to ignite, it can be compressed more and not combust before the spark.
when you use it in a car that doesn't need a higher octane, the fuel might not finish combusting before the exhaust stroke, and if that happens, you get worse fuel economy.