99% of concrete slab problems associated with cracks are related to improper compaction, because of soil that will not compact properly or lack of mechanical compaction. Although some building inspectors, cement contractors or engineers will not want you to leave the wire in the bottom of the slab it is the best place.

Think of the concrete slab as a beam and the pressure will be downward.

The area below the reinforcing has little strength and will open easily. 

The area below the reinforcing has no strength and will open easily. 

The more concrete above the reinforcing the sooner the reinforcing will come under tension.
The reinforcing has no strength until it is placed under tension.

Although pex tubing is not normally damaged by freezing it can damage the concrete if the tubing freezes. The pex can expand and crack the slab if the tubing is in the bottom of the slab the cracks will show in the bottom but will not come through to the top.