Do I have to cover pipes under slab?
Normally your drain pipes are ABS plastic having them bedded in the rock is fine.
If your supply is copper (I do not recommend copper under a slab) it should be coated so the corrosive effects will not cause pin holes.
I have had to replace several sets of copper pipe under a slab it is very expensive the houses were not very old maybe 20 years.
I would use cpvc or maybe pex pipe under the slab. I love copper pipe but not in or under a slab.