In a recent Geocaching newsletter, we were encouraged to log DNF's as they occur. A DNF tells other geocachers that the cache may be more difficult to find than anticipated or may be missing. In either case it reminds the cache owner to ensure it's still at the proper coordinates.
But once a DNF has been posted, it runs the risk of becoming a self-fulfilling prophecy. Other cachers may avoid the cache, assuming it is missing or too difficult. Cachers from out of town may not include that cache on their pocket queries to avoid wasted time. A particularly unique cache may languish while cautious geocachers wait for someone else to post a find.
Finding a cache that has been most recently "Not Found" adds an extra layer of satisfaction and sense of accomplishment to your caching experience. Go for it!