Anyone providing services to the public who is exceptional would greatly appreciate a little thanks you and the best thank you is a monetary reward for a job well done. There is no requirement or even guideline for most services off the ship. With that said, there is no actual written requirement as Rick pints out for a restaurant or porters at the pier or many other service professions. They do feed their families from the jobs they do in providing you a great vacation experience. When anyone, your barber, hairstylist, garbage man or anyone else providing you a service is exceptional you can make their day with a tip.
By the way, you'll feel better doing it as well. That's gotta be worth spreading a few bucks around for the smiles you recieve.