Usually the route is minimum time, but with regards to the NAT tracks on that particluar day. If one is to cross many (more than one) NAT track it is usually best to plan the route away from the tracks or to join a track. If many track are crossed one can get a clearance as low as FL 280.
Regarding the clearance from Iceland; it is given on the ground before a flight, however only upp to FL 230 or 240 depending on the direction of flight and higher when on contact with Reykjavik ATC.
Rrds Fresca