I went to AIDA's closing show; there were some people dressed up, but they were obviously people who were going to be attending the party after. I wore a skirt, but that's because I usually get semi-dressed to go to shows - nothing that I wouldn't ordinarily wear. So, wear what you want. Plenty of people were dressed very casually.
And the cast did stop at the stage door. There were TONS of fans, and a lot of security (which you'll probably see at WT as well, because of Brooke) and the cast obviously wasn't as accessible as they normally were, but they were absolutely gracious and friendly.
A work of art is an invitation to love.