We do both, but it depends on the cruise. Next year is a special cruise for us on the Allure, and I don't want to be boxed in with assigned seating. But there are many dining choices on that ship and we plan to dine in those alternate locations on most nights. We'll likely have a dinner en suite too. So I decided to not tie up an assigned seating for three people.
However there is something comforting about knowing where you are going to sit, and when the doors open you simply walk to your table and sit down and shortly your waiter hands you the menu as he or she says good evening.