I live and breath fuel quality issues in my job. Premium is recommended for the engine, and that is the gas I use. It may cost a little more to fill up the tank, but at about 10 cents a gallon differential, it's only about a $1.50 more per fill than using mid grade. You'd be fine with mid grade, and the car will adjust for the lower octane, but you do increase the change of potential engine knock and it may (not necessarily) cost you a smidge mpg- but that is really dependent on the fuel (fuels do vary in energy density, within a limited range). However, most gas has a minimum of 10 percent ethanol in it now (national renewable standards) so the difference in energy value of the fuel between regular, mid and premium is now marginal. The octane number if the primary difference. Given that, and the fact that the numbers on the pumps are "minimums", you may actually be getting a little more octane than posted -- so 89 may be 90 or higher, etc. -- but I wouldn't count on it. They won't "give away" octane like they used to when splash blending occured.

Eitherway, even I knowing what I know, I still use premium. If it's only a buck or two a tank difference, and you fill up 1 time per week, it's only 50 to $100 a year difference.