I don't mind the effort, since I like building wheels; and I really dislike the waste of throwing away perfectly good stuff. Overly cautious? I don't know.

I don't know how many wheels I've built since 1980, I guess maybe thirty or fifty. Also don't know how many of them were with old spokes; at least half, I guess. I've had a problem with spokes breaking on only one of them, and it was one with new spokes. Evidently I tensioned the spokes all wrong; I hope I learned my lesson. If a wheel is habitually braking spokes, then I'd guess some, or most, or all, of the spokes are getting fatigued; and there's no way to know; in such a case I would throw them all away. But if the wheel was built right, and there's nothing wrong with it but the rim is worn out or dented or something, I'll take my chances. No problems to report (so far).