Higher octane in cars that don't need it is a waste of money. You'll also end up forming a lot of carbon deposits if you run it continually.
Modern cars that use premium can usually run lower grade fuels without too much complaint cuz the computer/car is adaptable enough to pull timing etc... you just end up lowering performance, amount is about equal to the difference in the price between low and high grade... e.g. if low grade costs 10% less, you end up losing 10% hp.

I usually run 1 full tank premium come May-ish to flush out the crappy winter-grade gas.
Also, if you store your car in the winter, you should also run a full tank of premium when you take it out cuz of the extra additives etc to help flush out old gas with the fuel stabilizer.