We have given extra tips several times. On our cruise on the Carnival Valor...Lela and Edurado our servers in the MDR made the dining experience so much fun we just had to give extra. On our cruise on the Island Princess the stateroom steward made sure our ice cooler was always full and did a great job on our room. Our last cruise on the Norwegian Pearl, we found out that we could have the same people serve us our food everyday if we wished. Budi and Julie went above and beyond in their service, even serving us when they weren't on duty or on standby status so we tipped them a little extra.
I agree with the fact that a tip is for extraordinary service and not just, doing their job. Even though we pay the auto tips, there are times that some of these fine folks make our vacation just that much more enjoyable and deserve the extra we give them.