The hatching does follow the hair direction and the contours of the musculature. (Years of brushing and currying help here). In shadow blocks I would use my finest nib, dilute my ink, hatch as close together as possible in a direction at an angle to the hair direction. I'd fill in that whole shadow area, trying to create an even grey value. (Rather like laying in a underpainting shadow wash in watercolor) Then I'd begin, with a larger nib, darker in and follow the direction of the hair, sort of fanning out as I moved down the neck..so the next go round I'd fill in more. (Kind of hard to explain)
I'm attaching a rapidograph/overlay I did when I first started doing them. It's a better close-up of the line work, but I was not being near so fastidious in this. Also, it's much smaller. Probably 7x9...not near the effort or need to be so fastidious as in the 22x30 where uneven areas are pretty visible.
Hope this helps.
Equestrian Portraiture & Illustration