Katie, they just last week did away with the 'dress up' night on the three night cruise. They still have a recommended 'dress up' night on the four night sailings, but based on what I saw last January there were not very many people dressed up much.
I was kind of surprised by how casual the Wonder was overall (having been mostly on the Magic, with only a few sailings on the Wonder) The dressiest we will likely be is for a Palo dinner.
Other nights dinner will likely be polo and khakis for Steve & Trevor, and sundresses or capris for me. Not sure what Tara will do (probably sun dresses.)