I would say dry. It is easier to see the ends and to know where you have been. And it keeps you from cutting too much. Especially if you are doing it. It does not have to be blowdried. I usually do the clipping when I have done some twists. So, as I take them down, I snip the ends.
When I stop by one of the cheapie places, they usually trim it damp (they spritz it). They ask me how much I would like to trim off and I tell them. I never usually need more trimmed than a 1/4 inch each 3-4 months. And since I don't get many split ends, if any are missed, it can wait until my next trim.