We just got back from the Sun Princess and the service provided by all staff was outstanding. With this form of tipping, it's in everyone's best interests to look after each passenger very well. In fact, the staff are watchdogs for each other. If one isn't pulling his/her weight, others will ensure they're brought into line, since that behaviour will have an effect on everyone's final pay.
Just as a piece of information, we had an opportunity to discuss this with Marion in the Horizon Court. Staff are paid $50.00 per month - that's right - per month! Out of that is deducted $17.00 per month for medical insurance. That leaves each employee with an actual paycheck of $33.00 per month. They exist for the term of their contract on the tips from passengers. It's in their own best interests to give you the best possible service - and believe me, they certainly did for us!
Hope this helps a bit, Nick! I strongly suspect you'll be increasing your tips, not deducting!