Is your heart set on a strapless and/or backless gown and you happen to be a 34 DDD? You need to try on that gown without a bra, because that is how you will be wearing it on your wedding day. Pay attention to what the models in the photos look like. It’s very likely (although there are now a few sites with “normal” sized models) that in addition to being quite tall and slim, the model in that gown is an A cup or less, and she’s not wearing a bra.
How your gown will fit depends greatly on the foundation garments you wear. If you are trying on strapless gowns, consider if you will need a bra or bustier, and if that bra or bustier will be covered by the gown.
You will definitely want to wear your shapewear for every alterations fitting.