Hello Justin
Yes, this is true for QoS in general. QoS is there to ensure that packets get forwarded according to the priority you’ve assigned them. If there is no congestion, that means that an interface is capable of immediately forwarding any frames/packets that arrive. In such a scenario, no buffering or dropping of packets is necessary thus no QoS mechanisms will be activated.
For policing and shaping, however, this is slightly different. What you are actually doing is changing how you define congestion. Shaping and policing will define an upper limit to the allowed throughput even if that upper limit is less than that of the physical interface.
In the same sense, if traffic doesn’t reach those limits you set, you are not experiencing congestion, thus no dropping or shaping features will take place. However, when those limits are reached (you could call that congestion) only then will you see shaping and policing activated.
So both shaping and policing will kick in only when the configured limits are reached, otherwise, neither feature is activated.
I hope this has been helpful!
Laz