It was mentioned, but Shadow Fang is a little different from DOTs on other jobs since that skill is also your slashing debuff (I find it's best not to assume another job is going to do it for you, as you can never really rely on other players' skill levels) so you really don't want that to drop off. As a general rule, if I'm starting my three-move combo, and my DOT has 6 or fewer seconds remaining, I plan to end that combo with my DOT move. If it has more, I don't. You might clip off one DOT tick this way, but it's generally better than letting it fall off, especially if you're losing the slashing resist down.