I can only talk for Jedi shipping boxes, "kenner" as we used to flick through the boxes to pull out the few we needed at our local market shop, 1983-85 they used to just open the shipping boxes and leave them on the floor in rows, there were always a mixed bunch and after some time there was the usual unwanted jabba's hench men left and a few madine's, I remember a distinct lack of the key characters and stormtroopers, hoth and endor soldier's as I was army building ever back then. Can not say for earlier shipping boxes as they were always on racks when I got them, there was a tread on here some time ago by a chap who worked in a toy shop that might be worth looking up :?:
I can't comment on Kenner, but in my local toy shop back in 1978/9, they used to put the Palitoy cases on the shelves sometimes and let the kids pick the figures straight out of the box, and those assortments were definitely mixed rather than straight cases of one or two figures.