Just talked to my mom - she cruised on RCCL and Princess, ate in main dining room every evening. Her points:
1. As long as you are not wearing shorts or dirty jeans, nobody cares
2. There will be people in taxedos/long dresses
3. There will also be people NOT in taxedos and NOT in suits. Short dress is also OK.
4. There was a gentelmen wearing tax/bow tie and shorts. He was everyone's favorite.
5. My dad brought formal suit to his first cruise and never bothered to do it on other cruises. Wore nice slacks on formal night. Was still welcomed in dining room. Noone cared.
5. Pls. refer to ##1 again!