My girlfriend keeps hanging my compass on the wall. Is this doing it any harm?
According to this site there are only a few things that can go wrong with your compass:
Mechanically, it can become hard to read because of a cracked dome or contaminated fluid; it can leak, causing a bubble in the fluid which, if allowed to grow, will interfere with damping of the dial; or it can become “sticky,” a condition that prevents the card from rotating smoothly. “This last problem can be caused by a worn pivot bearing,” says Kaufmann, “or the card can ‘run aground’ for some other reason.” And even a mechanically perfect compass can exhibit deviation
The real problem is where your girlfriend is putting the compass not the orientation of the compass. Unless of course hanging the compass somehow causes the card to become irrecoverably stuck against the glass/lens/dome.
Any magnetic force close to the compass—even the energy around 12-volt wiring—can induce deviation, too.
So if she is hanging the compass right next to an inductive charger, in a welding shop, against your cellphones every night, on your fridge next to handy fridge magnets, etc. I might be worried... Otherwise you're probably safe.
Of course you swing/tune your compass yearly anyway right? In all seriousness I've never met anyone that did this for land navigation, but it is a best practice. It becomes more common for compasses used in ocean navigation, but even there it is declining due to the dominance and ease of GPS based navigation.
Don’t confuse deviation with variation, the difference between true and magnetic directions caused by the separation between the magnetic and terrestrial poles
Notes on tuning/swinging compasses:
Most of the literature is for fixed boat/ship navigational compasses because ships/boats travel longer distances than hikers without landmarks, have systems on board that might disrupt the compass, it is harder to lose a fixed compass, and your hands don't need to hold a fixed compass. Most handheld compasses aren't built to be tuned but tunable compasses will have some way to add little corrective magnets. Those magnets compensate for the deviation. If that isn't an option for your compass then you just have to build a deviation card and carry it with you. Depending on your compass you can use the declination adjustment to adjust for declination and deviation.
This site gives a good basic overview of the procedure. It was written for small boat fixed compass tuning but the principles are the same.
Compass adjusting is as much art as science. To adjust the compass on your boat, we must take the boat out on the river to swing around in circles. I will give you a heading to put the vessel on. Then I'll shoot a relative bearing on a distant object, using my polaris, and then compare the result to what the bearing should be. The difference is compass deviation. We will repeat this process for all four cardinal points of the compass. From that point I will begin to add equal and opposite magnetism around your compass, continuing to take bearings, until I am able to remove as much of the error as possible. Large ships require the use of two tug boats and a River Pilot to do the swinging around in circles.
Sometimes I need to mount small magnets to the deck near the compass but in other compasses there are built in magnets that are adjustable to "compensate" for the offending magnetism on your vessel. After the cardinal points are as "clean" as possible, we will then read the inter-cardinal error and record it. Only on a steel vessel with inter-cardinal balls is this error adjustable. So for most vessels I will only record this error, which should be minor, if the cardinal error is mostly eliminated.
I will provide you with a deviation card laminated in plastic for you to keep at your navigation station to use to correct your course headings to allow for the remaining compass error.
Most handheld compasses won't give you an option to adjust built in magnets so the best you'll be able to do is use the deviation card you create to adjust your course/bearings. On compasses with a declination adjustment you can use that, in combination with your declination adjustment, to simplify the corrections.