I agree with everyone above...there is something about dressing up that just makes one feel better. I usually just wear a jacket and tie, but if you like wearing a tuxedo, go right ahead!
Actually on my recent Carnival Elation cruise, I don't remember seeing even one person in a tuxedo. It could be that I just didn't notice, but I don't remember seeing anyone.
Either way, I'm sure you will really enjoy the formal nights.