We bought our DD's dresses at the
Disney Store when they were having a sale.
We had a 9AM BBB reservation and so it was just easier (and smarter if you ask me) to have them wear the dresses to the park that day. I didn't want to be carrying all the extra clothes all day after they changed.
It was absolutely no problem at all!!! I didn't see any other little girl come out of the dressing room for the 'big reveal' that you talk about?? Not saying it doesn't happen...I just never saw it.
My DD's felt every bit the princesses after the hair/nails/makeup and all the fun cute things the Fairy GodMothers do! Even the PP photographer was super great and fun!!!