ishe,
The INS/IRS may be used at any Latitude from 90 degrees North to 90 degrees South to reliably provide TRUE Headings and Tracks. It is completely independant of Magnetic Variation.
Within the areas where Magnetic Variation data is reliable and STABLE, the FMC calculates the local Magnetic Variation from it's internal database, and applies this value to the INS/IRS computed TRUE data to provide Magnetic data. It is commonly held that data is reliable from 60 degrees South to 73 degrees North, at higher Latitudes the Magnetic Variation is either Unusable, Unreliable, or subject to an excessively high annual rate of change.
Although a great deal of the higher latitudes (e.g. up to 80 degrees North)exhibit reliable Magnetic Data, the rate of change can be excessive, thus, the long term use (more than a year or two) of Magnetic Headings / Tracks from the database (currently based upon the year 2005) leads to too much difference between Computed Magnetic data and actual.
As mentioned in an earlier post, even within these reliable areas certain key-holes exist where the use of Magnetic data is not an option.
It would be a better world if we were able to abandon Magnetic data altogether, and go 'fully' TRUE, but this would then require that all aircraft be equipped with INS/IRS, a bit out of the budget of most C172 owners! Magnetic is retained (within the usable latitudes) for commonality between all aircraft.
I have recently been tasked by a FMC manufacturer for provision of an up-graded Magnetic Variation database, which compensates for the actual date and thus computed Magnetic equals actual Magnetic on any given day. Even then such up-graded aircraft would be incompatible with fixed year aircraft databases AND published En-Route Charts, and would only be compatible with those aircraft using Magnetic as a primary source (including AHRS). Even with time compensation the problem arises that the present rate of change, although fairly reliable forecast, also changes. About 5 to 10 years is the limit for such forecasts. The year 2015 is currently about the latest limit for reliable Magnetic predictions, but even then the North Magnetic Pole, after centuries of wandering randomly around Canada, is moving in a straight line at about 70 Km per year towards Russia (the new Northski Magneticski Polski).
A bit long winded, I'm sorry, but that's the gist of it.
Happy Navigating!
Regards,
Old Smokey