Reading online, they do say that some companies are better than others.
I got scammed at the airport on first arrival. I didnt go to the taxi stand and just grabbed a random taxi parked outside the arrivals door. Lost out on a lot of money (relatively). I would imagine that pre-arranged would be fine, otherwise, just make sure you get a taxi from the official taxi stand. There's a queue for you to stand in. When you walk out of the arrival doors, I think it's to the left about 20 yards (going from memory). There is a sign outside. Of course I'm talking about international arrivals. I've never flown domestically in the Nam.