This is a matter of personal preference and how comfortable you are with your cleavage and wearing strapless dresses/tops. I am a 38 DD. For my first wedding, I wore a strapless dress with a corset back. It kept me in place so you probably won't have to worry about pulling your dress up all night. However, there was a lot of cleavage and when I bent down to hug someone, a lot of boob in their face. I really wasn't a fan. So, this time around, the very first thing I decided was not only was I having my dress custom made, but I was having it made so I could wear a comfortable bra with it. No strapless bra, no corset, but a bra I could wear every day. They designed my dress around that concept and I was sooooo comfortable all day and night!
The dress should be fitted to you perfectly and thus keeping you in and the dress up. If you think you would be uncomfortable then add the straps. What you dont want to be is self conscious on your big day!
I am a 32DD and wore a strapless dress with corset top. Let me tell you, my dress did not move at all! At first, I thought I would be worried and constantly pulling it up but I didn't do that once. It held me in.
But you need to do what feels comfortable for you!