I always stood as a musician, but then concertinas aren't very heavy.
I wouldn't like to stand all night with a big accordeon, or handling a set of drums. As a caller all of my bands have been sitters and I've never made an issue out of it. I think it's horses for courses. Your standard general public village hall wedding do, sitting down is fine. Big lively festival ceilidh, I'd like to have as many standing and looking active as is practical.