Disabling auto system updates until you can verify there are no software defects which could adversely affect you is an excellent idea if your phone is your only device and you do not have a backup device, especially as there is still no full and reliable backup solution for GrapheneOS.
You also cannot go back to the previous version of GrapheneOS if you end up with a broken phone from software defects, which is both a feature and a risk. You can only hope and wait that the GrapheneOS project will release fixes for a bad update promptly.
Regrettably, GrapheneOS does not offer a security-updates only release channel like other projects and distros, which would minimize the risk of every update while maintaining security as the highest level.
I disable updates by removing network access from the system updater.
While I wait a few days or a week or more after a release to see if any defects are reported here and elsewhere that would adversely affect me, I also review the change notes and evaluate them. That way I can best balance my risk of breaking or crippling my device / usage from an update or new "feature" against my threat model in relation to the identified and patched vulnerabilities.
I also want to stop my phone from frequently contacting GrapheneOS servers which would disclose my IP address and to a greater or lesser degree my location by proxy. Disabling polling for updates is one easy way to enhance my privacy.
Users use their phones in different ways and have different threat models so YMMV. But this approach has worked for me.