Rough Plumbing Question

We are adding a bedroom and a bathroom in the basement in Maryland. My plumber mentioned that instead of using #57 stone, it was better to add pea gravel under the pvc sewer line, because pea gravel packs better. He also said that the old sewer pipes were laid on the ground without any gravel under them.

Also how much gravel should be placed under the sewer pipe and how much above it.

Thanks