You're required to have a chase under a slab. A slab walkway that breaks up landscaping for instance should have a pipe going under it.
If the wires are rated for direct burial, than you can put them in a trench. I agree that putting a conduit under the slab is a good idea for future use and to protect the wires.Can I just put the wires in a trench that will be under the concrete slab or do they need to be in PVC conduit?