I would even say "never". If you get sore, that means you didn't warm up enough - the best way to break your voice sometimes permanently. I always warm up my voice before any performance, I avoid shouting and screaming, and above all, people should not smoke after singing. I don't smoke (and never did) but I can see enough people smoking after singing and that's not good at all.

I don't see why harmonizing specifically would cause extra problems. XD But sometimes when I sing a lot, I don't warm up properly beforehand, and that can definitely cause issues. Or sometimes if I just push myself too hard (which I always do on the last performance of a show, because I don't care if I have a sore throat the next day).

I generally have to drink something, because my throat is dry, but other than that, I have no issue.