Theré are big differences in flanges, tread widths, and transition between them, so it isn't as cut and dry as scale gauging. I not seen a numerical measurement for any tubular. Maybe NMRA?
There is a descriptive process printed somewhere in Lionel literature to place an engine on a 0-27 curve, then there should still be an ability to shift the wheelbase around just slightly without the tread lifting or flanges bumping. I guess that would apply to any min. dia..
The gauge doesn't need to be exact from one wheelset to another on one truck. It would be ideal though..e.g with fast angle treads if there is a gauge difference, then there is a height difference to. But the tracking won't be really be effected that much as they want to stay as centered in the V-angle of the wheels seating.
Getting fussy would be better long term for wear, etc., but we are talking some minute improvement on bullet proof design 
Too narrow and you will hang up on guide rails in turnouts and uncoupling track centers(or fall into the rails). Too wide and you get binding and lifting /climbing on rails in curves.
