I always insist on cabs with visible meters or I negotiate a deal ahead of time based on what local knowledge I can get.

We also have an issue at the ferry dock where the limo drivers gather to pick up fares. Its a mandated $28 from anywhere downtown to the airport. Limo guys claim to be taxis (which is clearly not true once they lead you to their car) and offer trips for $35. Call BS and they drop to $30. I regularly tell them to get stuffed - taxis only (the fare is clearly posted on the window). Real taxis are regulated, licensed, and have consistent pricing.

When you get a choice, insist on a real taxi.