Those are my exact same thoughts on the matter. I don't mind the $10 a day service charge, and I do give a little extra the last day if I feel it is merited.
My waiter on my last Princess cruise told me that he is notified of which passengers have tipped automically. If he recieves extra from those passengers, he can keep the money. If he is gaven a tip by someone who has not done it automatically, he has to turn the money in.