I think they absolutely have to be optional for moderators.I can almost see uses for moderators being able to upload files (or to take some other "notified" actions) without creating a notification entry. Mods / owners are particularly likely to undertake setup and organization tasks that might otherwise create a lot of nuisance notification items.
I would not want members to be able to do so.
If this is implemented I think it should not be piggy-backed on the existing checkbox, if that one remains, but should be a separate one with a clear name and description. For example:
|_| Do Not Notify
Checking this box prevents the generation of web or email notification
to group members for this upload.