I don't do anything to the vent line. Since it is always a bit wet with oil not one single dust particle will make it to the tank inside, where the felt clunk would take care of it anyway. When I fill up, and the ventline overflows, it is clean again. I have never seen it to be dirty on the inside though.
I normally do nut fill up the tank that much, but sometimes it happens. When it does, it flushes any trapped particles in the line.