The "don't wear the band t-shirt to see the band" argument is ridiculous. It absolutely does not mean you love the show/band more than anyone else. That's absurd.
If you go see RENT wearing a RENT shirt with an Elsie key-chain, a pair of Mary Janes on and plaid pants, that's a little much and you're kind of crossing the obsessed stan line.
Your best bet is being comfortable. Don't wear a suit to see Rock of Ages, but don't wear flip-flops and daisy dukes to see Little Night Music. In my opinion, it depends on the show. I dressed a little nicer to see Chicago than I did to see Xanadu.
Into the Woods, Beauty and the Beast, RENT, Mamma Mia!, Dr. Jeckyll and Mr. Hyde, Evita (with Julia Murney), Hairspray (with Paul Vogt), Peter and the Starcatchers (with Christian Borle), Lion King, Altar Boyz, Legally Blonde (with Lauren Zackrin).