The INS needs to know where it is located initially so that "calibrate" its location. So for mechanical type INS its really like lining up your DI with your compass. You are telling the INS something that you know for sure. So the indication that you are given is then always referenced to the distance & position that you move away from your original set position. At each airbridge gate the lat and long are usually printed on the stand position so that you can either check or realign the INS BEFORE departure and not along the taxiway....
So to get back to your question about aligning to true north, the INS does not actually do this (correct me if I am wrong) rather it sets its self up in known space and then the minute deviations are calculated to represent a real change in LAT and LONG, so there is no actual magnetic influence on the system.
And for further explination (much better than my dribble) see the other topic posted on this forum called Inertial Reference System A320 in Tech Log, Luftwaffle & HPSOV has all the answers for you...
[This message has been edited by togaroo (edited 21 September 2000).]