Yes, they do deteriorate. But the rate varies so much from one to the next that you can't reliably predict a good time to replace them as preventive maintenance. Once in a while, shut off the gas and run the carb's empty one night when you are done riding. Pull the plugs next morning, lay them on the head with the wire attached and watch then while cranking the starter. If you get a nice fat blue spark on both plugs, the coils are probably doing well. Another check is to make sure the engine will still rev to the limit under load. If you get a puney yellowish spark or the engine starts cutting out early when loaded, it would be a good idea to get the coils tested on a proper testbench.
The reason for running the carbs empty and waiting 'til next morning is to avoid a funny surprize. If you have a combustible mixture going through the engine and your sparkplugs are sitting there sparking away, you could get a bit of a flamethrower effect.
