Carnival adds the tips to your account immediately. However, that does not mean that you automatically have to pay, even for poor service. You can go to the pursers desk, ask for a form to fill out to remove tips from any particular one , or all , if you were dissatisfied with the service.
However, if you were unhappy with one area of service, it wouldn't be fair to the others to remove all the tips--just the particular one you were unhappy with. When they first started that particular system, I didn't necessarily like it but soon found it worked out very well. Only once in many , many cruises have I deducted a tip from one individual staff member.Usually I end up doling out some additional for the good services.