I've never had any problems with used spokes. Some of mine have been re-built onto wheels a few times and have over 30,000 miles on them now. Jobst Brandt happens to use his same old spokes whenever he replaces a worn out rim and simply leaves his in place on the old hub while re-lacing. I really don't worry about even using mine in the same position on a hub... and, still I've never had any problems with breaking any spokes at the J-hook end.

As mrrabbit emphasized, evenly tensioned wheels are strong wheels - which distribute any stresses evenly among the spokes. Just take your time and do it right and you won't have to worry about a wheel later. If you notice that any of the spoke nipples are damaged (rounded) you would definitely want to replace them right away when re-building a wheel... otherwise you may never be able to tighten them properly.