OK thanks @featheredtoast and @marianord
About half the time when I do updates on servers, the next time I log in the MOTD is ‘A reboot is required’, so I had decided that rebooting would probably be necessary most times I run the update.
Very easy to have two playbooks, one which does dist-upgrade, autoclean and reboot and one which does just plain upgrade, no autoclean, and no reboot, maybe?
I tend to trigger these playbooks manually, and watch the output, so they are not running on a cron job or on a daily basis, which I would agree is risky in terms of rebooting in the middle of work.
Note that due to the way Discourse works with client-side Ember code, users can be in the middle of writing a reply when the server reboots and they will probably not notice any interruption. Reboots take less than 15secs usually in my setup.