I generally try to touch up at the factory angle, as less metal has to be removed. There's nothing wrong with reducing that angle if you have the patience and the sharpening media to remove the metal. The "Sharpie" trick helps let you know if you're sharpening all the way to the edge or just hitting the bevel. Bear in mind that some fixed-agle, clamp assisted devices are stating the angle at the jaw edges. If you have a wider head that protrudes significantly past the jaws, the angle is reduced. I like to sharpen as low as I can go without hitting the ferrule as RNB mentioned. Some like the secondary or "micro-bevel" the Lin mentioned and feel that "shoulder" gives the edge some backbone.