I NEVER use any DLC if the code is hidden behind a leaflet or behind a grey "bar". I keep them mint. And I collect rare unused DLC cards (such as Lost Odyssey, Too Human, The Last Remnant...)
I ALWAYS use them if the code is already visible on the card (what would be the point of not using these ones?!).
Examples:
http://www.flickr.com/photos/krasnajazvezda/7704717740/
http://www.flickr.com/photos/krasnajazvezda/7704705080/
http://www.flickr.com/photos/krasnajazvezda/7497785684/
http://www.flickr.com/photos/krasnajazvezda/7497783908/
http://www.flickr.com/photos/krasnajazvezda/7497824506/