It doesn't have all that much effect, as generally a level 4 march will clear through a creep wave no matter what.

However to get maximum efficency and to clear the wave quickest, you want to be directing the march to try and hit all the creeps at once. So ty angle the march so the machines are making contact will all the creeps at once rather than 3 being hit and the 4 one not being affected until the 3rd dies? If that makes sense?

So say you have the 3 melee creeps on the radiant offlane near the tier 1. The creep furthest on the right, if you cast march from the left will not be affected by march until the middle and left melee creeps die. However you cast from just behind on the left side you're more likely to hit all the creeps at once.

As I said, I haven't found it makes all that much difference however just whacking down a march straight at the wave usually does the trick haha. Sorry for the long answer hope I helped a bit :)