True, you do not have to run Sonar while running latency monitor to tell if it will work.
That said, you should be able to have sonar running and be mixing etc and not have latency monitor detect an issue.
Once I had an issue where arbitrary clicks occurred. So I ran latency monitor with Sonar until I heard the click then looked to see the driver with the greatest latency. It was the mouse.
This pointed me in the direction of the fix.