Is there any reason to back up these system-created certificates? I understand the need for backing up keys and user defined certificates, but is there any reason that we should back up these certificates that are system-created?
MS_AgentSigningCertificate
MS_PolicySigningCertificate
MS_SchemaSigningCertificate34284BF7CE6208DC9D835E9F45ADD94F0757439E
MS_SmoExtendedSigningCertificate
MS_SQLAuthenticatorCertificate
MS_SQLReplicationSigningCertificate
MS_SQLResourceSigningCertificate
Your first certificate needs to be backed up if you are moving the MSDB between instances.  https://support.microsoft.com/en-us/kb/2000274

In short, its not typically required to explicitly backup those certificates.  Particularly if you are backing up your system databases correctly.