Oh that kind of blocking. Why don't you make regions that are just a bit bigger then the tower size, and make a trigger to move to the center of the region, and when they enter the region, they're ordered to the next. That way, the units don't need to move to the center, but will automatically be ordered to the next region. Also when they enter the same region again, it'll won't effect them, since they will execute the same order again, which doesn't have any effect at all (or replaces the old order).
Just watch out not to make the region too big, or you have a chance that the units will choose a shortcut.