I would replace a cap and rotor if the metal contacts look distorted or pitted at all - not if they just look a little dirty. There will be tiny bits of the contacts in the cap missing and the rotor may look like some one had tried to weld on its tip. I have replaced these items about every 5 years or 60K miles on recent cars.

Replace your wires when they look anywhere as if a mouse or rat might have been chewing on them, or have any visible cracks at all. Short of replacing them, clean any dirt off the wires, the outside of the cap, and the ignition coil, with a gentle solvent. I have not replaced wires absent a specific symptom.