To expand on the "during hours" idea even further, it's not so much ' during hours' as machine utilization. NR (at least some versions) had the ability to set it so all cores are used once you log off, but that only seemed to work for the user account that this setting was configured in (it should work that way for ALL users who log into the machine by default). That would enable conference room machines (that get little actual use), to be utilized through most of the day, and if someone logs in, the rendering is paused until they log back off.
As for regular user machines, most of our users leave themselves logged in all the time, if the screen is locked and there isn't significant CPU activity, then all the CPU cores are fair game.