Actual pathfinding is often overkill or even unwanted, in that the waves of things attacking are better off actually attacking through barriers on what is otherwise the best path, based on static features, you know … lanes or what have you. If someone can build across a lane and block it, do you WANT the units to actually turn around as a line of lemmings and start going through the woods to the other lane?
All that asside, id think that individual units would not have paths, the path would be for the spawner and inherited for everything it spawns, or be for the entire lane (to the destination, target of attack, homebase, etc) and would be super cheap to network it.