Choose your North Arrow wisely; it implies Magentic or True North.

I always chose my north arrow based on style. However, my advisor reminded me to be more careful, as explained here: http://www.cavesurvey.com/north_arrows.htm

Basically, a north arrow with two sides indicates true north, while an arrow with one side indicates magnetic north. Choose wisely.

On a related topic, you can calculate the magnetic declination of your location here: http://www.ngdc.noaa.gov/geomag/