I hate to disagree with Mel but when fishing with a floating line I would not dream of using a leader as short as 5ft. I tend to go for the length of the rod tapered leader. I find it helps turn over and keeps the fly away from the line.
On the other hand when fishing a full sinker or tips I fish a short leader of about 3ft, as I want to keep the fly down.
I tend to agree more with what Juro said.