But it could be useful to know beforehand how far out compass is if you one day need to set a course without the chart plotter?
Yes you can load the boat down with spare GPS units running of small batteries as a backup.
But seems to be a cheap precaution to find the deviation?
So if you ever need the compass - some warnings about what could make it unreliable could be OK.
mariners found there way for hundreds of years without the use of gps. In my opinion if you can't navigate without gps you should not be skipper of any vessel.
Dosen't take much to learn the basics and correct your compass for error. Then you must remember to keep electrical things like portable radios etc. away from the binnacle or compass location. I set a portable radio close to the compass once and wondered why I just passed what appeared to be a duck blind in the fog. Turned out the radio was affecting the compass. No harm done