There are also some "reality factors" that may call for fudging the chainline. One of them is present on my own mountain bike: I have a Ritchey Z-Max 2.35" rear tire, and if I shift into my inner chainring, the tail of the derailleur rubs lightly on the tread knobs sometimes, due to the size of the tire. Shimming the bottom bracket out a few millimeters and readjusting the front derailleur accordingly would fix this, although I haven't seen the need.
Another "reality factor" is that some derailleurs, particularly when mounted to a large-diameter seat tube, swing inboard as far as they can and scissor shut on themselves, or hit the frame, and still are not in far enough to get the chain onto the inner ring. Again, scooting the BB/crank out is a solution.
Personally, I don't fret about chainline beyond a practical level.