You should backup the central Repository (and alternate repositories if used) daily. Work with at least three generations of backup media (tapes, CDs, etc.) so you can always backtrack through the files of the last few days with no effort. It is also recommended to hold the last weekly/monthly backup at an alternative location to protect from local disaster.
If an RCS file is mistakenly deleted or damaged, you may simply restore it from the latest backup. In addition, the CS-RCS setup files (located on the Repository root) may be safely restored if damaged.
Note: Many organizations also backup the local user's working area. This is unnecessary with CS-RCS, as any significant work is always checked-in to the Repository.