It depends on what the paint is like on the wheels. For any factory painted wheels I'd think a wheel cleaner would work just fine, I like Eagle One A-2-Z (in the orange bottle) because it's not too harsh. As you mentioned, Simple Green should work well.. but I'm not sure how harsh it is, maybe dilute it a little. Another really good & safe wheel cleaner is P21S, although it's hard to find and slightly more expensive.
Any aftermarket wheel or repainted wheel I would just use soap & water. Once they're clean, put a coat of wax or synthetic sealer (ex. Klasse, Zaino, Blackfire, etc) on the wheels inside and out. That will make them REALLY easy to clean from then on because most of the brake dust will wash right off with a hard stream of water from the hose rather than sticking to the wheel.