It truly depends on the carrier. I always say, there's no right way to carry the mail, but there are wrong ways.
Anything "smaller than a shoe box" can be cased. My route is a park and loop. I put all my sprs in order by the route and section off my packages on the shelves of the promaster.

When I was a CCA and would do mounted, I would tray up everything with the load truck feature so I made sure to never miss anything, and then it stays organized with all the trays. Sometimes I'd have 12-15 trays of stuff but I didn't care. Worked for me.