I agree Krispy, everyone will have different experiences.
My commentary is because I live here, I manage a hotel and every time a taxi comes to drop off clients, I ask in front of the driver how much they were charged. I do this with AeroMovil
taxis from the airport, the collectivo and the yellow city taxis. I do this because I report the taxis that charge my clients the incorrect fare ( take down their sitio number and car number), as well as to spread the word people are checking to assure the taxis are charging the correct prices, as well as for my own personal information.
When I call cabs for my clients, I tell the driver where the client is going and make sure they are being charged the correct price.
The cab company phone number is 223-2285 which is a radio taxi service of yellow city
taxis.
None of these things are a rule, as there will always be someone who tries to rip someone off, but it is pretty good rule of thumb for travelers to follow.