I'm always encouraging people to change their gear cable outers when they're having shifting problems. I change mine at the rear derailleur curve every six months. Changed it recently and the shifting didn't really improve, it remained vague and sluggish so last night I treated the bike to a full set of new gear cables and.... what a difference! Gear shifts are light and positive both front and rear. It's amazing how you become accustomed to deteriorating performance and allow it to creep up on you. The cable outers wear slowly especially if you've a 10 speed cassette so are changing gear a lot. Muddy wet conditions cause even faster deterioration of performance. It's an easy job to do as long as you follow a few simple guidelines and you have a good cable cutter or you get your LBS to make up the right lengths of cable outer for you. A new inner cable isn't so important from a wear point of view but I usually change mine if they're tatty. Sometimes I fit new to the rear and recycle the old rear one to the shorter front position.