I would recommend leaving the tip on your sign and sail account to avoid any confusion (Carnival now (or at least has on all my recent cruises) automatically adds it to your bill unless you remove it manually at the pursers desk).
Easiest way - then if the service is exception - cash in hand is ALWAYS a great plus to them!!!!
I always tip my Steward $20-25 when I first meet him and let him know that I intend to also leave the other tip in place. This truly motivates EXCEPTIONAL service.