Make sure that at least one of you sits in the front of the car so that if the meter doesn't get switched on, you can bring the driver's attention to it. In fairness, every taxi ride we took in Fes, the meter went on with no questions asked. No journey came to more than 15 dh, most were less than 10 dh though I tended to round up to 10 dh anyway. Moroccan driving is basically insane, so maybe close your eyes until the end of the ride. Seatbelt optional...
Only twice were we 'ripped off' by taxis, in Rabat and Tangier, but that was for a maybe 10 dhirams each time, so you've got to keep it in perspective. Did make me appreciate the honesty of the Fes taxidrivers though.

