opening in a slab

There is not a simple catch all answer for a generic question such as this. You need to know the loads, in whose jurisdiction is the slab located, existing slab reinforcement, what is the intended use/purpose, is the slab structural, is it on grade, required safety factors, how big of a penetration is needed, what is the shape of penetration, subgrade conditions, are there any loads being applied across the penetration, what are the parameters for failure, etc.

I think you would need to supply a lot more information regarding the features of the slab/reinforcement, use/loading, size/shape of penetration, and local building requirements.

For some non-structural on-grade floor slabs small penetrations for some components like small sewer pipes do not require any additional reinforcement, the WWF is usually considered sufficient even with penetrations. However, in post-tensioned floor slabs it is a different story as the reinforcing cables are structural and should not be penetrated. Additionally, state and local building codes vary and should set the minimum acceptable requirements. You should consult the UBC (or IBC now) and contact local building department.