I no longer carry spares. I figure that is sensible given that:
1. You can limp along with a broken cable. It is pretty easy to rig the derailleur with the broken cable to your most used gear.
2. In the US and Europe you will likely never be more than a couple days ride from a bike shop.
3. Cables rarely break and pretty much always show fraying for a long time before they do (you do have to know where to look and actually do it though).