If your distributor cap is bad you might notice some hesitation when you accelerate or even rough idling.
Your gas mileage could suffer as well. I recommend replacing the distributor cap along with the rotor when you get spark plugs replaced or when you replace the spark plugs yourself.
A good thing to do as a tuneup and preventative maintenance if you can afford to do so is replace the spark plugs, the spark plug wires, and the distributor cap and rotor at the same time.
You'll notice a huge difference in gas mileage and performance in your engines power.