Demoting master to slave without an rsync...is it safe?

[Chris Redekop <chris@xxxxxxxxxxxx>]

I have a master+slave set up using asynchronous streaming replication.  If I do a graceful (-fast) shutdown of the master, and then promote the slave to master, my understanding is that I should not have any data loss.  At that point in order to bring the old master back up as a slave the docs say I should do a base-backup etc....however I've found that simply setting recovery_target_timeline='latest' does allow the old master to start back up as a slave, and everything appears to be happy, all data seems to be synced properly.  My question is, is this safe to do?  It's a very attractive option as it removes a significant amount of load from the master server for planned switch-overs (maintenance/upgrades/whatever).  Thanks!