I don't remember ever seeing a printed spec on countershaft or mainshaft endplay, but I've never seen one that needed any shimming beyond what the factory did with the original shims, which are not selective, meaning all the engines got the same shims on the assembly line. As long as I have what feels like a "reasonable amount" of play when I assemble a bottom end and I can manually shift through all the gears by spinning the shafts and rotating the shift drum, I figure I'm all set. Now, If I were assembling something a little rarer, say, a 70's vintage Spanish bike, or anything old and European for that matter, I'd probably be a bit more fussy and actually take measurements.
1985 Tri-Z 250
1985 ATC250R