Don't be gulled by slick oil company marketing ploys about the benefits of premium fuel: few new cars really need it and those that don't won't run any better from using it.
There's no mystery to it. Just take a look at your owner's manual; it will tell you the manufacturer's fuel recommendations. There may also be a sticker on the gas cap or even on the instrument cluster under the gas gauge.
Whatever it says, abide by it. You're only wasting your money by burning premium fuel in a car that doesn't require it. Higher-octane (91 and up) gas burns more slowly, and will actually give poorer performance when fed to engines that were designed to burn regular 87-octane fuel.
But the reverse isn't always true. What happens, in fact, if you use regular or even mid-grade gas in a car that really does need premium? If the car in question is a late-model one, nothing that will cause any permanent problems. The computer will adjust the ignition timing and other engine parameters to compensate for the lower-octane juice. You may notice a slight falloff in acceleration, but no engine damage or drivability problems should arise.
But with some older, pre-computer cars (model year 1981 and before) you could have a problem. For example, a Sixties-era "muscle car" with a high compression ratio must have premium fuel to avoid deadly engine knock (pre-ignition), which occurs when the gas and air inside the engine's cylinders ignites before the piston reaches its firing position at "top dead center." When that happens, the explosion tries to force the piston down when it's coming up -- and that puts enormous strain on engine bearings, connecting rods and the relatively fragile aluminum pistons themselves.
Unless you want to ruin your high-compression engine, premium fuel is an absolute must in such cases. You may even have to add a can of octane boost to each tank in some cases to bring the fuel up to spec. In this case, premium fuel prevents engine knock because it is less volatile and hence burns more slowly than lower grade gas. Thus it is not as susceptible to pre-ignition.
However, even today's "ultra" premiums come nowhere near the octane level of the leaded premium that was available 30 years ago. In those days, octane ratings of 100 were common; today 94 is the best you can get -- and the octane level is raised not by lead but by the addition of "aromatics" that may cause problems in older engines.
Fortunately, very few cars still on the road today have high-compression engines that need such fuel. The handful that remain have usually had their engines rebuilt with lower compression pistons to run on today's lower-grade gas -- and the others can avail themselves of octane boosters readily available at auto parts stores.
You should not buy octane boost, however, for use in an emissions-controlled car with a catalytic converter. Octane boosters may foul the converter and eventually plug it up. Besides, no factory-built produced since the early 1970s needs the stuff anyway. You're just wasting money and buying the advertising hype.
There is one thing, though, that could cause your late model, regular-fuel car to need a higher grade gas: age. As an engine gets older, carbon buildup on the tops of the pistons effectively increases the compression ratio -- which in turn means you may find the car knocks when you use anything but mid- or even premium-grade gas.
This is a normal condition and nothing to worry about. Your engine will run great for many miles to come, so long as you feed it the stronger juice. If you wish, there are ways to flush the engine and purge the carbon from the tops of the pistons, but this service is not cheap, and the pistons will eventually get a coating of carbon all over again anyhow. It's typically cheaper and certainly less hassle just to spring for mid-grade gas.
With this exception, you should stick with the fuel recommendations of the manufacturer and avoid being sucked into the trap of paying extra for something you don't need.
If for some reason your car still knocks, the ignition timing is probably off or you need a tune-up. Don't crutch the problem by going up to higher grade gas; have the car looked over by your mechanic to determine what the problem is.