I've never paid any attention to the Ambient Occlusion setting on the Properties tab for lights. For doing realistic renders (hopefully without too many artifacts), is this something I should be using? I'm trying to understand whether this is something "artificial" that technically shouldn't be allowed by the other light settings if the brightness were realistic, or....?

One reason I ask is because I feel like I'm not getting sufficient shadows with dynamic hair, particularly against the skull, even though I have all hair set to Casts shadows and Light emitter. But perhaps I'm doing something else wrong.... I have skullcap set completely invisible to everything.

In the Math room : CHECK Opaque in shadows on the hair node.

In the hair properties : UNCHECK Light Emitter, leave Cast shadows checked..
Re-render

And washing away shadows is a signal that you could have too many lights in your scene.

Thanks, Vilters. No, I don't have too many lights, and shadows are not washed away. It is only at the hair roots that I expect more shadows than I'm getting; everything else is fine. And unchecking Light Emitter? that improves speed, but it doesn't give improved shadows, just makes things less realistic....

essentially, Ambient Occlusion is meant for IBL lights only.

It should not be in the materials any more, it should not be on infinite / point / spot-lights, and it should not be around in IDL environments.

But of course, one can find some artistic needs for AO, for instance when switching off light emitter to take the surface out of the IDL process.

In my opinion, IBL/IDL are good in soft lighting but tend to ruin the shadows and shades that give depth to the scene and shape to the objects (and translucency to scatter). Direct lights do the opposite. High GC exaggerates the workings of IDL and reduces the workings of Direct Light, low GC does the opposite. So the art is in the balancing of those elements, like photographers are balancing natural light and additional flash.