When I go to Coz it is usually by buying a travel package with flight, hotel, and transfers all included, and it also includes having a representitive from the travel company meet & greet me at the airport, give me a welcome package, AND tell me that he needs to meet up with all of us staying at the resort (different resorts different meeting time) to give us an orientation of his or her duties, when to get ready to leave for the airport, sell us tours etc. Every one of those meetings has had one common message, and that is that you don't need to tip the cabbies, buy no proper explination as to why not. I think it's has to do with the fact they are well paid plus maybe some jealousy, or maybe the resentment that the taxi union controls transportation on the island, but I don't know.
My person advice & how I go about it is to tip those who in my mind gave good service, no matter what their job. If a taxi driver is friendly & informative he gets a tip, but if all they do is drive the car no tip. When it comes to hotel staff, restaurant staff, dive boats etc I probably over tip, but I won't if they simply gave me the minimum in service or weren't friendly. I don't consider myself the least bit wealthy but by their standards I am rich, and I am happy to share my money with them IF they go out of their way even a little bit over what their job demands. Being employed in the tourist industry isn't a guarantee of being tipped, it still requires that they are friendly & helpful when dealing with me.