I'd replace them if you notice and physical damage to them as well. Just tipping over is enough to pinch the clutch cable.

They are very easy to replace though, and don't require a lot of mechanical know how. So if you do feel the need to replace. It should be easy enough.

 

They can break if they are not routed properly and wear in spots that they shouldn't.

If you want to be preventative, take it off and give it a look, make sure there's no spliters coming apart and such. Toss some cable lube on it and put it back on.

My clutch cable broke a while back, probably around 16,000 miles or something (I don't remember what I had). But my bike had previously been down, so it's possible it was slightly damaged at that time.