Depends on your design and/or setup. Here is a rundown on Caching-Only DNS servers.
They do not host any zones and are not authoritative for any particular domains. Caching-only DNS servers start with an empty cache and add resource record entries as the server fulfills client requests. Now the information is available in the caching-only server cache when answering subsequent client queries. A caching-only server can be valuable at a site when DNS functionality is needed locally and creating a separate domain or zone is not desirable.
I hope this helps explain it a bit more for you.