I'm sure during action strays would go in some less desirable places than expected- I would say you are safe to strew a few for effect- who is to say you would be wrong to? Enjoy your fun part of this stuff-
Although it's definitely not a Vierling mounted on this vehicle as it's only an SdKfz 10, I think it's safe to say that you just kick the spent cartridges off the bed whenever the opportunity arises.