You can eat in the buffet if you like. The "formal nights" are a mixed bag. I have seen folks in jeans and sweatshirts in the dining rooms. Not just on formal nights, but every night. I was sure that they would not let them in, but they did not even blink an eye.
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08