Best rule of thumb is use the lowest octane you can without getting detonation. Start with the factory recommendation (87 for a GT), and then go up only if the car's pinging.
Using higher octane gasolines without need will not increase performance, may even hurt it and is possibly over the long run bad for your engine.
my car pinged on really hot/humid days with 89, so i have ran 93 since last summer with no problems...don't run anything higher than 93 without a tune for it
It depends on what you've done to your motor. If you are stock or nearly so, 87 is fine unless you are pinging and then you will want to run higher. If you don't have any pinging, you are wasting your money if you run 89 or 93. If you've advanced your timing or are supercharged, turbo, or nitrous, you definately want to run 93.
I have to run premium now with the Timing Adjuster, but before that my car ran best on 87 octane. Stock our cars are designed to run on 87 and unless your mods require premium, you are most likely wasting your money.
I run 91 oct. I have found that my stang runs the best on 91. It's generally a better grade of gas as well. I try to use Chevron any time I can, because it has a great cleaning agent in it.