My wife's '06 GC also has a rough idle, yet I have '05 and do not experience the same rough idle, at least not yet. I do notice that once it has an oil change it does idle smoother. I try and use the STP fuel injector cleaner every 3 fill ups just to try and keep the injectors and the fuel system somewhat cleaner.
As far as gas, we both fill up wherever the gas is cheaper that day, but usually the same stations, so I don't believe it's a gas issue. As far as I'm concerned gas is gas, they all get it from the same terminals. I know I've heard it before that it differs from one brand to the next, but I worked at a gas station in my college days, and the owners also owned a trucking business where they delivered gas to independent gas retailers, basically what they do is they would call there terminals and find out who had the lowest prices, then they would buy from them, after all do you think the no name gas stations have their own refineries?