If they allow you to then its good to have a few options
I took her in a pair of pretty jeans and a white sleeveless top, a romper suit type thing, and also a pretty summer dress...also some with no clothes on, just a nappy.
I wouldn't be too tempted to buy new, simply cos I did but to get it to look like it fits on the day you'd probably need to buy the size she is about to grow out of...and then never get to use it again!