I've had a pile of grab-bag spokes that I had previously removed from wheels. Different lengths, different diameters... I built a wheel using them quite a while ago. What a mess that was Never again.

If the spokes were relatively new, and STAINLESS, and fit the new rim, then I'd probably re-use them if it was just unbuilding one wheel and building a new one.

I would not bother reusing galvanized spokes. It would also depend on whether you're wishing to use something like double butted spokes, and what the originals are.

I'm now breaking spokes on my front wheel of unknown age and history (36h, straight generic spokes). Now broke 3 outbound spokes (15 left to go). It appears as if the heads are not well seated. Anyway, I'd be more reluctant to reuse generic spokes than name-brand spokes (DT, etc).