Former Broadway orchestrator here with ties to this show!

Ultimately, like others mentioned, wear whatever you like and are comfortable in. If you were going to the opera, that'd be more black tie or formal. This is the national touring production, so you can really get away with anything. I've seen it all, from full suits to spaghetti straps. Just make sure it's comfortable to wear and sit around in for some time as you'll be sitting for a while (Act 1 is normally around 75 minutes, Act 2 closer to an hour).