Been thinking about this thread off and on through the day.
One of the lures of electronic devices is that they yield very precise numbers. The chart plotter says the next waypoint is on a bearing of 113 degrees. So staring at our fluxgate compass readout we try desperately to steer a course of 113 degrees. But that's a fool's errand. Wind, waves, currents, measurement error, and our own inattention cause us to steer a course that is hopefully somewhere near 113. What we need to do is steer a course that averages 113 and constantly adjust that bearing based on leeway, currents, and so forth.
Then there is the precision of a traditional magnetic compass. At least on my compass, the lubber line is about 4 degrees wide. That means I can only read a compass direction of about +/- 5 degrees if I'm lucky. So does it matter if I have my compass swung and a deviation card created? At what point does deviation even matter?
Unless we are racing on a very short course, course swings of 5 or 10 degrees don't really matter if we are reasonable navigators, because we will periodically determine our position and adjust the course to hit our waypoint.
I guess the upshot of this, is to know if on some point of sail your magnetic compass is way off due to local magnetic fields, however, if the deviation is less than a few degrees, don't sweat it. And if your GPS, Mag Compass, and chartplotter all show different headings, but they are within 10 degrees of each other, don't sweat it, until you get real close to the rocks. In which case, you need a lot more navigation skills than simply blindly following a compass course.