If you have indexed shifting (STI, ERGO, etc.) it's best to replace the shift housings when replacing the cables. The condition of the housings is just as important to good shifting performance as the cables. Same can be said for brake housings but they seem to last longer than shift cables and housings. Excess friction inside housings will cause sluggish shifting, especially those shifts when the cables are moving away from the shifters and the derailleur springs are pulling the cables through the housings. The short housing found at the rear derailleur is subjected to more dirt and water and are usually the first to go.