I have never replaced one unless there was a problem with it. I am with Ian if it ain't broke don't fix it. It would be interesting to see a side by side comparison if you could measure the intensity of the spark produced at the spark plug. I just look for a nice dark blue spark and a loud snap. I don't know any other way of checking them. I know you don't want to be in the circuit even with a weak component.

I have no faith in human perfectability. I think that human exertion will have no appreciable effect upon humanity. Man is now only more active - not more happy - nor more wise, than he was 6000 years ago. Edgar Allan Poe