If you have a turbo, or supercharger, or are experiencing Detonation ("Knock") you need higher octane gas. Octane rating is merely a measure of resistance to detonation and is needed for high-compression engines. No matter what the oil company advertisements say, there is no other benefit to 'premium'.
If the engine don't need it, you are wasting your money.
http://www.cartalk.com/content/colum...August/03.html
http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm