Only reference i can find is in my 92 softail manual and i knew that i had read this but it took a wee while to find
Disc rear wheel spacer pack MUST be assembled on the Sprocket side. If the spacer pack is not assembled on the sprocket side there will be no end play and the wheel bearings will seize during vehicle operation,resulting in personal injury
It may be that in the past someone has read this statement and applied it to all rear wheels they have done to ensure they are always safe although i do not think it matters which side of other types of wheels that the spacer pack is fitted
i did the wheels on my softail this time last year and read through all my books throughly and i have a disc wheel on the rear but perhaps all the rear wheels on all my bikes will end up with the spacer pack on the sprocket side
Brian