When and where do I have to worry about accounting for sensor "noise"?
Switch bounce is a physical issue that can be cleaned up with filtering. Since the sense states are very simple, only very crude (time-based) filtering is required to clean up its signal.

Other mechanical sensors that can exhibit similar characteristics are bump sensors, tilt sensors, and accelerometers. Bump and tilt sensors are also very simple, and similar filtering will clean up their signals.

Accelerometers on the other hand react to very minute perturbations, so they may require more advanced filtering based on the application they will be used in; orientation sensing can use time-based filtering, whereas things such as motion tracking including dead reckoning will require one or more layers of calculus in order to clean up the signal.