Even though you don't HAVE to dress to the nines, my wife and I find this is part of the fun. How many opportunities do you have to wear a Gown and Tux and have Professional Photographer take photos for free. you only pay if you buy them. Considering getting a formal picture of yourselves at home. You would be charged just to have them come regardless of liking them in the end. So with that in mind I urge you to dress up. Take the plunge.
For example on Elegant Night #1 I wear my Kilt with full battle array (minus dagger which I have trouble carrying through security) and my wife has some evening wear. On Night #2 I wear a Tux. And yes this takes up about 1 third of 1 suitcase but boy have we some nice photos and memories and once got on the Ships Video.
It's Vacation you can get crazy. its OK