Tipping is a very gratifying part of the cruise for me, because I know that this money is helping to makes someone's life better in an under-developed part of the world. Most of the tip money they earn is sent to their families, who they only see a few months of the year.
Yes, you are paying a lot for the crusie. But you don't stiff the waiter at a fine restaurant just because the steak dinner you ordered was expensive. When you compare what you would tip to stay at a hotel and eat out three times a day, the tips on a cruise ship are minimal. I'm glad that Princess adds it onto the bill automatically. It's charged to my credit card at the end of the cruise and I'm not running around looking for bills to stuff into envelops.
While we're talking about tips, be sure to take some extra dollars to tip the people taking your luggage at the ship terminal and to tip the tour guides and drivers on the excursions you take.