everyone once. to look at it another way, the line has to pass by it once. lines are generally short enough that a tower on the right side of the screen will be more effective because the line will pass it twice. the last guy will have moved out of range by the time the guy at the head of the line comes back into range. so, i found towers along the top (which can reach both the entrance and exit) and along the right (which must be passed twice) to be the most cost effective.
in my picture above, the tower on the far left is the most wasteful. a 