Tourist,
I agree with Orange. I've been to CR a number of times and have never had a problem with any of them. I have had a couple of drivers stay with me and my group most of the night, buying them beverages and then collectively giving them a nice tip at the end of the night.
One small problem I did encounter -- the language barrier -- was quickly resolved by showing the driver the card of the hotel I was staying at.
I also agree with the statement about flagging down a vacant cab. Doing this near the DR at night, every once in awhile you'll get some people more than willing to open the cab door for you but I just tell them politely

that I'm a big boy and I can open my own door.
I think your best bet is to find an English-speaking driver, preferably one associated with a hotel. Of course, you can search and find on this board many opinions on a variety of drivers.