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.