I can honestly say (getting old though - the memory is failing!) that I haven't seen jeans in the MDR at dinner.
Have seen someone get turned away at breakfast for a sleeveless muscle shirt and a baseball cap.
On the first night, you see a lot of walking shorts for men. Most men, I'd say are business casual (dockers, nice khakis etc.).
I agree with Manuel, jeans are hot, and not quite appropriate for the warm weather.