I think it depends a lot on what you find under the black paint. Polishing will make them look good temporarily, but if original plated finish was bad, then polishing will remove more of the remaining protective plating, exposing bare steel, and the spokes are going to start rusting. An option might be to strip and polish them and coat them with clear paint.
I would not worrry about issues with stretching...you're going to have to true the wheel anyway. Just check afterwards to see if any of the spoke ends are protruding excessively through the spoke nipples and touch them up with a Dremel or similar tool.
Kevin