Tipping above the standard gratuities is an option each passenger makes.
While we may differ from others, our practices is that if a crew member just does their job we do not feel compelled to tip extra. For service above and beyond we tip commensurate with the additional service and attentiveness.
My norm for Butlers, Room Attendant, Favorite Bar Tender, Waiter and Assistant waiter is $5 - $10 pp per day.
We only tip at the end of the cruise and never tip al la carte.