Our understanding is that the account agreement for a joint certificate of deposit does not have to be signed under the FDIC deposit insurance rules. Are there other reasons for recommending that we obtain all co-owners’ signatures on a joint certificate of deposit account agreement?

Yes, we recommend that you obtain all owners’ signatures for a joint certificate of deposit. Obtaining all of the co-owners’ signatures will unquestionably bind all of them your bank’s terms and conditions for the CD.  Ultimately, you might be able to argue that just one owner’s signature is necessary to bind all of the co-owners to the terms of the CD, but the conservative approach would be to obtain all of the co-owners’ signatures when they jointly purchase the CD.