Magnetic declination doesn't change much over short distances. However, it's possible there could be local variations when going into the mountains so it's best to do the calibration in this case. More importantly, make sure you give the H about 10 minutes of extra time sitting on the ground after boot up so that it can update the GPS.
True, but a review of a magnetic declination chart would show an east/west "tilt" of the declination lines when looking at their north/south orientation. It's something we need to maintain an awareness of. More important for longer trips probably. For shorter distances like 50-150 miles perhaps GPS has greater importance, although it's best to have both in sync. The time and effort involved to do that is small.