If it’s daytime, and you are travelling within the medina or the New Town, insist the meter (le compteur) is on. It starts at about 1 dirham 60. If they say it’s broken, tell them to stop and get out. There are lots of other taxis.
The only ones you negotiate with are the Grand taxis. Petit taxis only take three people. Taxis can be difficult to get during rush hours, 8 – 9.30, 12 – 2 (that’s when they change over drivers too) and 5 – 6.30.
At night, they charge 50% more. Rides in the medina are generally about six dirhams, from the New Town to the medina generally about ten dirhams.
Make sure you have small change.
Getting a taxi from the medina to the airport on the meter is about 25 dirhams, but getting one from the airport to the medina is negotiable. Published tarif is about 60 dirhams, but you may have to pay 100, depending on number of people and amount of luggage and time of day.
After that, how much you tip is up to you.