To my experience on most SQL Servers there is no backup of the transaction log.Full backups or differential backups are common practice, but transaction log backups are really seldom.So the transaction log file grows forever (until the disk is full).In this case the recovery model should be set to "simple".Don't forget to modify the system databases "model" and "tempdb", too.
A backup of the database "tempdb" makes no sense, so the recovery model of this db should always be "simple".