I don't know, but it would cause all sorts of problems.
First you would need to take into account the magnetic variation for each map and time period. Magnetic variation changes slightly over time, so to be meaningful you have to model that change over the period covered by the sim.
Charts should be marked with the amount of variation and how much change would occour each year.
If you were going to that detail you'ld probably want to magnetic deviation for individual aircraft and have compas correction cards displayed in the cockpit.
How useful this sort of information would be in the sim would be debatable.
If your flying as the RAF, I'm not sure how much map and compass work will be needed, Hopefully we'll be vectored onto our targets by radio then we'll just take fixes on the local pubs (landmarks) and follow the roads and railway lines back to base!
Cheers!