With magnetic compasses (as opposed to electronic compasses) it just means that you need to check the area's declination on-line rather than counting on "historic" or on-map values. The best place I've found to determine declination is
http://www.ngdc.noaa.gov/seg/geomag/declination.shtml (note the
Compute your declination link in the first bullet of that main page.
It is also yet another reason not to draw mag N/S lines on my maps. Some of the other reasons are that I don't like drawing big ugly lines on my maps since I tend to goof up a line here and there, and I'm too lazy to do it. The adjustible declination on my compass, using true north on my GPS, and orienting my maps to true north pretty much take of it for me.