mysql strict mode should be supported
System requirements still list mysql strict mode disabled as a requirement (http://docs.whmcs.com/Version_6.0_System_Requirements).
MySQL is starting to make strict mode the default in version 5.7 ( http://dev.mysql.com/doc/refman/5.7/en/server-options.html#option_mysqld_sql-mode ).
MySQL in strict mode prevents a lot of SQL that are actually incorrect or may give the wrong application results so enabling this mode will mostly likely highlight some underlying problems.
The application changes to get strict mode correct are usually trivial. I'd suggest WHMCS start using it in a development so that eventually this requirement can drop, hopefully before a stable MySQL-5.7+ version comes out.