I'm a big fan of NCL and their dress code. The formal nights are optional, but you can never be too dressy. There are folks dressed to the nines, like the couple we cruise with and there are those who are cruising to relax and don't want to have to dress formally, like my husband
My advice is to dress in what you feel good in and don't worry about other people dressing down that evening.
We sat at a blackjack table, after dinner and a show on formal night, with a gentleman who looked quite dapper in his black tux and another who looked just as handsome in his Tommy Bahama shirt and Dockers. Neither looked out of place, and quite honestly, no one cared what anyone was wearing
Enjoy your cruise!