xtrabackup backup grants?
Your backup solution for MySQL User Grants is fine.

I have been advocating doing that for years:


Mar 24, 2013 : MySQL export user with semi-colon &quot;;&quot; on the end
Dec 24, 2012 : Backup and restore &quot;mysql&quot; database
Jun 13, 2012 : Fastest way to move a database from one server to another
Apr 12, 2012 : Cannot GRANT privileges as root
Apr 09, 2012 : Mysql users deleted
Jun 26, 2011 : Restoring an old backup to latest MySQL release


Also, please notice that Percona created pt-show-grants and xtrabackup. It is curiously funny that pt-show-grants has not been integrated into xtrabackup already (Hey I could be wrong. After all, a physical backup of the mysql schema should be very adequate for a Backup of Grants). Perhaps, one of the following is the reason:


Business Decision to keep Text version of Grants separate from the Backup
Potential Security Reasons (should anyone get a hold of the Backup)
No one ever thought of it before


Going back to you original question, copies of the User Grants in SQL and the my.cnf are sufficient.