An old thread discussed Compass & Magnet Storage.

But I'm wondering...will prolonged compass exposure to the wrong metal (say, on my keychain, next to Leatherman Micra) do anything other than cause errant reading while they're adjacent to one another?

Thanks.

The metal close by will modify/amplify/substract the magnetic fields of the earth or artificial fields close by. Normally they should not be a problem unless you are trying to use the compass.
If one of the close by objects is magnetic, and the compass is continually moving within its field, it may effect the magnet in the compass.
Normally the magnet in the compass will loose some magnetism over its life anyway, but this shouldn't usually be noticable for decades or longer in good quality compasses.

I havent had any problems with any of my compasses from loosing accuracy from nearby objects. I do however watch what I do with any magnetic tools that I use nearby. I regulary use magnetic tools, and they get put in the back pockets as I'm working not the front pockets where my compass is.

Not really. Worst case with extreme exposure to another magnet it might demag or reverse the polarity of your compass. But those odds are so long they are not realy worth considering.

I had always heard that storing compasses next to ferrous objects (attract compasses) is OK, but it is important to keep them away from magnetic fields (batteries, anything electronic, etc...).

Here is a clip from a Silva web site. It does indicate that ferrous items can mess up a compass. I would think this is for long-term storage rather than packing during a single outing (less than 2 weeks).

Reversed Polarity - from http://www.silva.se/files/compass.html

Golden Rule - Any make or type of magnetic compass, should be kept well clear of magnets and magnetic fields.The SILVA compasses are manufactured in the non-magnetic compass factory in Sweden.

Cause - Magnetic fields exist around, some car seat belts, electrical circuits, etc. and RAPID movements within inches of these items, or even ferrous (containing iron) metal, an iron gate or fence for example, may affect polarity of the needle.

Effect - The needle either becomes sluggish and slow to settle (it may appear to stick and look out of balance) if its polarity is partially reversed, or may reverse its polarity completely. In the latter case, the "North" (usually red) seeking end of the needle will point South.

Cure - Quickly stroke the "South" pole of a strong magnet outwards along the "North" end of the needle. Repeat vice-versa. Compare with a compass that is known to be correct.