We have two DDs. One wore her Cinderella dress and glass slippers and the other wore her jeans and t-shirt. As a Mom, I think all the pictures are cute, so I think as another poster said, it should be the child's decision. DD7 (with costume) was absolutely beaming in her photos, but it may have had more to do with the fact she was actually meeting the princesses than because she was in costume. But she felt beautiful and regal and loved every minute of it. We changed her into street clothes in the bathroom afterwards, and her happiness was worth the extra trouble caused by carrying the change of clothes with us.
If your daughter is "prissy" like my youngest, dress her up if it doesn't break the budget. If you don't want to haul the extra outfit around with you, you can always rent a locker. Have a great time.