He doesn't, for a good reason.
Every valve job is diff, sets the valve diff height, the valves themselves come diff lengths...so which are you using ..then the guides can be in slightly different 'to and fro' by .002-.008 or wherever they end up..the machine they use matters there.. if if the guides are drilled by hand... the sloppy guide 'oval' allows the bit to follow the the direction or wear.
Many factors. Take your heads you just got out of the machine shop...put all valves in... then lay the edge of a steel straight edge or aluminum ruler across the stems...
Look for some not touching the ruler while others do. That's what I'm talking about above.
