Should I exclude mysql database from multi master replication?
Since the mysql schema is usually manipulated with GRANT and REVOKE commands, there is no need to let the mysql schema replicate. Why ?

Someone could hack privileges in by means of INSERT commands into mysql.user.

Such hacks are possible because I wrote about it before


Oct 28, 2011 : MySQL: trying to create root user
Sep 12, 2011 : How do you gain access to embedded MySQL database via localhost command line?
Mar 06, 2011 : Installing default MySQL Tables in Windows (mysql_install_db)


If these hacks were done on a Master, a Slave might intercept them. The new grants on the Slave would not be active until someone restarted mysqld on the Slave or ran FLUSH PRIVILEGES; on the Slave.

So, to answer your question: exclude it via the my.cnf