We are going on our 19th cruise next weekend and my DH has always worn a nice suit and never felt under dressed. We take his suit in our carry on luggage and turn it in the first day for dry cleaning. It is returned the next day before Formal night. I would guess about 15% or less wear a tux. Once on board, you can go to Formalities and rent one for the week if you change your mind.
Teresa and Larry
#36-Carnival Splendor 9/16/12
7 night Mexican Riviera
#37-Allure of the Seas 11/11/12
7 night Eastern Caribbean