It is just going to create arguments. This is a number that can be different at different libraries. LOC number is fine as it is just one library, but DDC number can possibly be different for many libraries depending on the libraries purpose and the cataloguer making the decision.
I think we're supposed to enter it only when it's printed on the release. At least in American books it's usually included in the Library of Congress cataloguing data, alongside with the LoC codes.
Agreed, it should only be entered if the book itself displays it.
Indeed, after posting this I left work and thought about it and realised that there is sometimes CIP data on the verso and this should be allowed. There need to be some restrictions on it. Also on occasion the CIP data will not be printed but there will be a referral to the national library for the data, it needs to be decided if this referral means we can add the DDC from the referred library.