I wore my Rainbows to a Jack's Mannequin show this summer and had no problem, but then again, J's M is more mellow, so I think it really depends on the artist you're going to see.
I don't see wearing flip flops to a show as that big a deal. Now, I wouldn't wear them to a bigger show that I knew was going to be more chaotic, but I think some of the comments on here saying you should
never wear flip flops to concerts are kind of absurd.
Honestly, I think it would be kind of ridiculous to
not wear flip-flops at something like an outdoor music festival in the middle of summer. I wouldn't want my feet to be all hot, nasty, and sweaty in sneakers
I just think it depends on the situation; there's no correct blanket answer.