Things have changed quite a bit over the last 20+ years my wife and I have been cruising. It also depends on the brand of ship as well. Back in the early to mid 2000's, we always dressed up to the max for formal nights. I always wore a tux and the wife a gown. As the years have passed, things have really gotten relaxed onboard most cruise lines. My recommendation would be to dress it up if you want to and as long as you have the capacity to bring your items in your luggage allowance! We are doing a 28day Scandinavian and Voyage of the Vikings cruise in early July, and I will wear a sport coat and tie for formal nights.