Fly's correct to an extent; changing intake/exhaust components can cause the factory ECU's to make minor changes to it's Air/Fuel Maps, but not nearly enough to properly keep the car's Ignition, Timing and Fuel functions smooth and in balance. In some cases, the stock vehicle runs too lean/too rich throughout the RPM range. Keep in mind this happens when you do heavy modifications to the vehicles intake/exhaust components. When I say that, I mean changing the air intakes, headers, running test pipes, and full exhaust. Usually doing all of these bolt-ons can cause your car to experience some of the issues mentioned above.
However, if you just get an aftermarket exhaust for your Z, usually you'll be fine with just exhaust, or an intake kit. Anything in addition to these performance modifications and we recommend getting the car flashed. We're still working on a Reflash for the 370Z, and we'll keep you guys updated.
Hope this helps!
-Jon