Hmmm, food for thought. My instinct is to say an unknown cost center should be stored as Null, not assigned to a bogus cost center. To me that's the definition of Null -- we don't know. Maybe your way makes it easier for OLAP tools and you are approaching this from a practical rather than theoretical POV. In environments where DBAs increasingly deal with new feeds, acquisitions, new labels, etc,, the need to report on unassigned, irregulars, not applicables, etc. goes way up. In some circles it's a must. Like I said, I've known people who came to regret their initial stance on storing nulls in FKs.
