If a tourist is staying outside Manhattan and in one of the other boros or even Westchester or NJ it is important that you have directions for your driver since they probably don't have navigation systems or know the neighborhood you are going to very well. Directions can be googled or mapquested or whatever website you prefer. Just print them off and have them ready when you get in the taxi.
This evening I was walking to my car when a taxi turned on my street. I rarely see a yellow taxi in my neck of the woods and it was going very slow so I knew the driver was lost. As I got closer to the taxi the back window opened an three women asked me how to get to Layton Avenue. Fortunately, they weren't too far out of their way and I happened to be going in that direction so I told them to follow me and they got to their destination. I then had to direct the driver back to the bridge so he could get back to JFK.
Anyway, while you may think a taxi driver should know their way around all of NYC, the truth is they don't so having directions will help them and you because that meter continues to run the longer you are lost!