Consistent logical backup of databases that use MyISAM and InnoDB engines
With mysqldump you can only safely use --single-transaction if all your tables are InnoDB, otherwise your backup is inconsistent.

If you have the requirement for a hybrid backup, then you need the lock-tables on all tables in the backup (default), which will be safe for all engines.  It's also worth mentioning that the default options will make sure your backup is safe, you don't need to turn any special flag on.

Note: If you do have a hybrid mix, perhaps look at xtrabackup.  It will only be locking during the MyISAM phase of the backup.