Vat you are somewhat right. The higher octane fuels are harder to ignite, so are less likely to preignite in a higher compression engine, that is why high compression engines need it, but the lower octane fuel actually has a higher energy content. That is why a car that does not need higher octane can actually make LESS power and get worse gas milage if you put 93 in it.
21mclcar, that is wrong. Engines with higher compression do need higher octane fuell. WIthout it, they will ping and could be damaged. Modern cars though mostly have knock retard that is advanced enough that if you put the wrong fuel in it can correct the timing enough to prevent preignition, while loosing power. Some cars actually say to use whateer you want and it will just make more power on higher octanes, like cars with the supercharged GM 3800, the comuter just adjusts timing and fuel trims to prevent preignition on lower octanes.
That being said, the 05+ Mustang GT 4.6 has such low compression it would practically run on water. If it's stock, I'd use 87 and nothing more. My car is tuned for 93, so I use 93 from whatever name-brand gas station is convieniently located (usually BP).