I've only shot a concert three times; once at my school, where I was allowed latitude, once in a nightclub that had a live act which loved the attention and once at a gig where the stage hands actually allowed me to step up onto the wings and shoot from behind the curtain. He could see I wasn't just faffing about and was very kind and an audience member actually kept a space for me and waived me into front and centre. I didn't take her up on this offer as I suspected alterior motives.
But if it's a free-for-all, a long lens and small stepladder are very helpful! If there is a lot of action onstage, it can be very difficult to get isolation. In this circumstance, I personally feel there is even less 'ethical correctness' in simply deleting other performers who have spent years learning their craft to be deemed "irrelevant" by some photographer. Rather, I feel it is better to try to anticipate a moment of interaction.