I'm cheap, so whenever I remove a set of cables in replacing brake levers or shifters, I save the one I removed from the rear and later reuse it on the front. That's two cables for the price of one--such a deal. Does everyone else do that, too? It occurs to me that it means I'm probably bending my brake cables at a slightly different angle or something and maybe theoretically increasing the risk of breakage.
On the other hand, I haven't broken one yet, and the risk seems very low to me. (Come to think of it, I've never heard of anyone breaking a brake cable, ever, anywhere.) Not a safety concern with derailleur cables, unless there's something I'm not thinking of just now.
I should add that I end up messing with my bikes so much that the cables are seldom more than a couple of years old before they get shuffled around--I toss any frayed or rusty cables on bikes that I acquire. And I only do it on my own bikes.
Am I going to hell?