On a link that never experiences saturation (zero output drops), does a QoS policy affect traffic at all?

I'm working with all flavors of Cisco gear (3560 / 3650 / 2960 / 3750 / N7k / N9k), reading through End-to-End QoS Network Design.

Consider two links, both uncongested (defined for this purpose as zero output drops):

1. default config, no QoS. Plain FIFO.
2. QoS applied with multiple queues, classes, and other stuff like WRED/CBWFQ.

Again, both links so far report zero output drops.

How do these links behave differently, if at all? Is QoS active even without saturation, or does it only take effect when buffers begin to fill to a certain capacity?