Are the values shown in the compass deviation placard implemented within the sim?
As a pilot in real-life, I like to fly VFR using pilotage and dead-reckoning; using the deviation figures are essential in determining the actual course heading for a particular route leg.
I don’t know if those numbers are accurate, but there is a toggle in the assistance settings for gyro drift.
And if they are, do they change as Electrical devices are turned on and off on the Panel ?? 
I honestly never paid much attention to those 2/3 degrees, it’s hard to keep the Directional Gyro in synch with the magnetic compass and then you won’t fly that precise anyway. Even if you would take this 1/2 degrees into consideration, the wind might be slightly off so it is still a guess work.
By the way, you would mainly use the Directional Gyroscope during flight and only use the magnetic compass to keep the DG in synch? In which case you don’t need to take deviation into consideration at all, only when realigning the DG. Set the DG to magnetic north and not the local compass north.
Be aware that the compass swing is often performed with pitot heat ON if the aircraft has one, therefore even when not flying in icing conditions the pitot heat must be on for accurate reading. Not sure if this is true for every aircraft but the ones I flew it could make a 10 degree difference in some quadrants.
