When I was filling in at our quarry, a lot of times the drivers who would be taking stone to the blacktop plants, would get a pre-load the night before. Clean stone is probably more favorable since it won't "soak up" any potential rain fall. I have seen one or two loads get frozen in the beds during the winter months. One was a pre-load and the other, the trailer's tarp broke I believe, and they had a 2 hour haul, the wind blowing over the load was enough to freeze the top of the load.... always fun.
Off-road trucks are a different story... Cat anyway, frowns on leaving an off road truck loaded over night... but there's a lot more weight in a 775 than a triaxle I suppose too.