I've found that dry burning is most effective if done early in the coil's life and regularly, before too much gunk is built up. This allows you to burn it off with just a few sets of pulses, versus having to pulse the coil a large number of times. When the coil needs extended pulses, the wick (silica) seems to dry out and fibers start breaking around the coil, and the chance of scorching the rubber grommet in clearos increases.
I recommend washing the coil off after dry burning as critical to get the best taste.