For one if your string and cable's have never been changed ,I'd go ahead and change both just to get a fresh start .I wouldn't change one cable without changing both at the same time .If your string show's wear, it definetly won't hurt to change it on it's own as long as the cable's are fine ,but being that you have one bad cable ,I'd change both at the same time .Also as Extreme1 say's you can get a teflon slide that will improve the wear on your string and cable's and also eliminate's some noise issue's as well.

nubo