eagle 1 makes a good point. If you're using a sinking fly, a straight piece of mono works really well. Since it's all the same diameter it sinks better. I prefer to use flourocarbon for this because it sinks better than mono. I do this mostly with very small jigs or "bead head" flies. The weight of the fly/jig will turn over the leader.

If you're throwing a big wind-resistant fly like a popper, though, I'd rather have a tapered leader.