It is considered a best practice to enable the Disable Thread Level Security option. When this option is selected, users will be unable to configure security settings for each Discussion Thread. Instead, Discussion Threads will be restricted based on Document Type User Group settings (i.e., users that cannot retrieve a document will be unable to access that document's Discussion Threads).
If you disable thread level security, users will not have to update Discussion Thread security settings every time a new user or user group is added to your OnBase solution.