the spokes take a fairly hefty cold set at the hub end in most builds. The potential problem with re-using spokes in other positions is that they will take a second cold set.
If you are really worried about this, sort the spokes into two groups (or four for a rear wheel) as the wheel is disassembled and use them again in those positions.
But really, you don't need to worry; you can cold set a decent spoke (by the amount required in most builds) maybe a dozen times or more before the spoke is liable to fail directly. Cold-setting good spokes a second time doesn't really hurt them any more than doing it for the first time.
Just as with a first use, the important thing is that the wheel is properly stress-relieved after building.
Decades ago, one of the more thoughtful LBS proprietors I have ever met showed me what he did to test spoke quality in a simple ad-hoc fashion. He gripped a length of spoke with two sets of pliers so that there was about 1/4" span between them, and flexed it back and forth, each time getting about a 90 degree bend in the spoke material. Thus each reversal was much more severe than several cold sets during a build would be. Good spokes, he said, usually survived about a dozen reversals or more, whereas bad spokes didn't.
Knowing what I know now, a lack of ductility in the material and/or the presence of any reasonable-sized inclusions in the steel, and perhaps an absence of a work-hardening characteristic all contribute to premature failure in this test. Such things can also compromise the longevity of spokes, or the chances of being able to re-use them, too; for a simple test, it is likely to sort the wheat from the chaff rather well.
Anyway, I've built enough wheels with previously used spokes to know that they can be perfectly reliable. I'd have no hesitation in re-using DT or Sapim spokes (I understand that they are made from the same type of steel from the same source) and other stainless steel spokes are likely to work OK too. There are a few types of (cheap) stainless spoke that are more likely to give problems anyway; wheels built with such spokes are only reliable if they are thoroughly stress-relieved, whether they are being used for the first or second time.
cheers
