Rather than deface the coin and ruin the coin's beauty.....
For those coins which can be activated and of which I have more than one coin.....I usually activate them with our geonick and then place them in caches locally. Most of the seroius collectors seem to avoid keeping these activated coins for their collection. So far, all of our activated and trackable coins are still travelling from cache to cache.
On the other hand, If you have a Moun10bike coin and placed it in a cache, it would quickly end up in my.....uh.....someone's collection. But, you wouldn't want to deface it just to keep it moving!