Hi, it's quite a tough question. As many things in rendering lot of things are situational where you adjust on the fly.
Generally not much (under .5) or at all, and I don't really see cases where you should go ever higher but it's just personal opinion (It's not dumb if it works).
When I was making VN prototypes, my go-to workflow was :
-Load that gorgeous custom skin you passionnatly edited
-Use Victoria 8.1 eyes (sclera/cornea/moisture...) as a base of work
Why Vic 8.1? Because it's overall well balanced maps/shader wise (as far as Uber/PBRS can go), not sure there is better (havn't seen yet but Im not up to date). I still not had time to look at Victoria g9 ones.
Now I always done speed rendering (IrayCam+portrait HDRI as main light source) and common problem is lack of reflection in the cornea (ie "dead fish" eyes). Look at
this post for slight reflection push in the cornea.