The stack of gears on the rear axle determine where your chain line should be. So if you want good shifting to them all with equal chain angles to the high and low sprocket, then shoot for a chain line that hits the middle within a few mm. If you know you ride more on one half of the cassette, then lean your chain line a little more toward that if shifting is satisfactory enough all the way to the other extreme.
It's more a try it and see thing than a absolute, IMO. If you are shifting well enough without and wear is okay, then how much more perfection do you need?