There are two issues addressed in the OP. One is positioning (where are you standing) and the other is where you are facing (what are you looking at).
I agree with Cindy that the position should be the best one if the netman has a play on the ball (you could cheat the other way on a 2nd serve, but I don't, personally).
The OP seems to be speaking more about where the netman should be focusing. I first concentrate on the server, since knowing where the serve is going is the first priority (way, way before worrying about the opposing netman) since I am going to change my positioning depending on where the serve is going. As the next logical extension of that, calling the service line is the next priority. After the serve has landed in the box, then and only then do I concentrate on the netman. Of course I do agree that at that point (but not before) the opposing netman is a higher priority than the server.