Whether you have symptoms or not, its always good keep up with the maint. of the car. I change my air filter about every 10-15K miles. I had the power steering fluid and the transmission fluid flushed at 30K miles. They also replaced the trans fluid fliter at that time. Get the tranny flushed somewhere other than the stealership...they rip you off with that one. Every 30K the dealership recommends cleaning and regapping the spark plugs as well as performing the fuel induction service which entails cleaning the fuel injectors, putting in fuel system cleaner, and replacing the fuel filter. It seems a lot of people dispute cleaning/regapping the plugs and the fuel induction service, but I notice a difference in both gas mileage and performance when I have those things done. The fuel induciton service is pricey, about $120, but worth it.
A true "tune-up" calls for replacing the plugs and wires and inspecting the coil packs, but GM's plugs and wires are supposed to last to 100K miles.
Jeremy