I believe there's no such requirement.
Backups can be taken online and completes very shortly.
There is no need to take the system off line, and the impact is minimal. A dump of select database tables is performed, and a couple files are copied from the file system. It will not impact mail flow.
The above post is correct, there is no need to bring the system offline, even during peak jours, and the CPU usage is negligible.
As always, you should do scheduled backups daily.