Should have a backup of the cert periodically.
I don't worry about the SMK, as this can be overridden, so it's more if you plan to restore this exact server setup again without some snapshot.
For the master key, the old backup would not work if you needed to restore it to get to the cert, but since this master key is in the master db, I'm not sure it's a concern for backups. You can take one if you like. More, having a cert backup is the important thing. You can always restore the cert under a new master key and it (the cert) will work with the DEK