Taxis (actually large vans) on Providenciales are metered, and rates are regulated by the government. On the family islands (i.e., the smaller, outlying islands other than Provo), taxis may not be as strictly monitored, so it's usually best to ask about the fare for your trip in advance.
On Provo, taxi rates are metered and are based on two people traveling together, but each additional person is charged extra. Fares for children are only half price—but always ask first. You may also be charged for more than two bags per person. Unless you have a rental car waiting for you at the airport, you will be taken to your resort by taxi. If you're using a taxi as your primary mode of transportation and you find one you are happy with, get the driver’s direct cell number. You will have to call for service later on; taxis don't hang out anywhere except the airport, especially late at night. And remember, drivers are a great source of information about the islands, so be sure to have them fill you in on what’s what. It's customary to tip about 10% per ride.
Renting a car is preferable, as there are only a few taxis available on Grand Turk, North Caicos, and South Caicos. If you want to get around Salt Cay, a taxi is really not necessary. Ask your accommodation or tour operator who might be around to greet you, and know that you can cover most of the island on foot.