I doubt there was a standard distance. The only reason to be concerned was that a brakeman could make it between two cars on the roofwalks/running boards/whatever they were/are. Other than that, I think the distance you mention just "happened", mostly based on the dimensions of the couplers and their shanks.
On a slight different note, EMD E's and F's were spaced 36" apart measured at the end of the car body (from Kalmbach's "Cyclopedia, Vol. 2"), and lightweight passenger cars were about 30" apart (from the Davies UP passenger car books).
Unless, of course, I'm wrong.