I don’t know but I’d guess it’s not too common. Sculpting is way way faster to create and refine concepts, most studios have always worked like that and the only change is that now they’ve gone mostly digital and clay + scanning, or rapid prototyping, is only involved in rare cases.
Additionally, should a portfolio or demoreel reflect such workflow?
I certainly don’t care about the workflow as long as it produces good looking and technically solid results (good topology, UV layout, rigger-friendly etc)
Solid box modeling skills aren’t bad though, but you only really need them to build base objects for concept sculpting IMHO. Anything that requires more complex forms is either hard surface and needs different, more precise methods, or it can be sculpted (and sometimes you want to sculpt hard surface as well, anyway)
What I’d look for is good skill at rebuilding on top of concept scultps, and having a good eye for topology that works, avoiding too many n-sided vertices (there are many ways to terminate or add loops and not all are equal) and so on.
Lastly, I was a little confused about what you said in relation to topology around the neck.
Are you saying that if I’m planning a displacement map I can add the forms in Zbrush and simply have topology similar to what’s in the left image below, as opposed to what’s in the right one?

I’d use the topo on the left, yes, but with a lot more geometry. Here’s the neck part of a head I’ve just finished yesterday to show you what I mean.

Also note that most of the edge flow is continued through the jawline like this, so edges run at an angle, about 40-50 degrees, to the shape of the bone. But it’s okay, because that line isn’t that sharply defined anyway; and when you open the jaw, you want to make it appear to slide under the skin and push vertices out that are on the neck in the closed jaw base shape.
I did not include shots of the concept sculpt but you can see that it has a vein on the neck, too, and the place where wrinkles are forming when the head is turned down or the jaw is opened to the max.
I could also make him pretty nice swallowing blendshapes with the adam’s apple moving up and down this way. And the entire head is like 9000 polygons altogether. Granted it’s a relatively young man, an old face would probably take some more work, more edge flow changes, and probably more polygons too.
Here’s some more high quality stuff worth checking out, from Clay Osmus:
http://cgclay.blogspot.com/
Pay attention to the fleshiness of the blendshapes in particular. Pretty hard to do with low poly models.
All af the above only apply to mostly realistic, detailed models that you want to animate, of course. If you’re only going for still images or cartoonish designs then it’s all quite different again.