Failback to old master

Hello,

I swear I have read a couple of old threads. Yet I am not sure if it safe
to failback to the old master in case of async replication without base
backup.

Considering:
I have the latest 9.3 server
A: master
B: slave
B is actively connected to A

I shut down A manually with -m fast (it's the default FreeBSD init script
setting)
I remove the recovery.conf from B
I restart B
I create a recovery.conf on A
I start A
I see nothing wrong in the logs
I go for a lunch
I shut down B
I remove the recovery.conf on AI restart A
I restore the recovery.conf on B
I start B
I see nothing wrong in the logs and I see that replication is working

Can I say that my data is safe in this case?

If the answer is yes, is it safe to do this if there was a power outage on
A instead of manual shutdown? Considering that the log says nothing wrong.
(Of course if it complains I'd do base backup from B).

Thank you,

M.