A backup is always a good idea. How much data loss are you willing to accept if things go sideways?
Method #1 Have an automatic Discourse backup. Mine runs daily to S3.
Method #2 Manually run a backup from the Admin /admin/backups.
Method #3 Create a snapshot of your instance where it’s hosted. DigitalOcean, for example, has a simple way to do that in the Admin. This is a bit slow, though - about an hour in my case.
Method #4 If you can also automatically backup your instance, do it. DigitalOcean runs weekly backups, for example.
So, to answer your question—if it’s convenient, time your upgrades right after a backup has run, or do a manual one.