I tip according to the fairly standard guidelines from other cruiselines, with extra for outstanding service, and less for poor service. I also try to remember to tip the favored, or most-used bartenders either daily or at the end, since HAL doesn't add the customary 15% to bar bills.
We totally agree with Thulewx about tipping. Even though HAL has a "no tipping" policy the crew have always worked extra hard to please. Treat them to at least a normal reward......or more for exceptional service, which they gave us on the Rotterdam's inaugral sailing and then again two years later. We will be interested to read other opinions.