Orvis sez:
http://www.orvis.com/intro.aspx?subject=2202
Honestly, I don't put this much thought into it. I generally stick to 5x, and move up to 4x when the flies become big, leaded, streamers.
For droppers, its 6x because I want them to sink ASAP and I don't want to think about the line hanging down from the dry hook, although I usually don't mind the top as anything but an indicator.