As an aside it is also interesting to note that most modern airliners only have one magnetic compass - ie the one in the middle between the forward windows.
The displayed heading on the pilot's display is derived from true heading (from the aircraft inertial system - accurate to tenth of a degree or better) and the addition of variation from the nav system's computed position and a database of world magnetic variation. The experts tell us (and I think they are correct) that this gives a consistently more accurate magnetic heading than the old slaved compass system.
Whilst the likelihood of a modern airliner having to fly on the magnetic compass alone for heading information is fairly remote, a magnetic compass complete which current deviation card is still a mandatory fit!
Finally, I think you will find that residual deviation on a direct reading compass must be three degrees (not the pop group or glidepath angle!) or less after the compass has been swung.