PIEty is the best secodary stat for WHM.
WHM only has two issues you have to overcome.- MP Management
- Healing through debuffs/%healing
The strength of your Cure I/II does not fluctuate significantly with allocating the 30 stat points. So don't fret about under-healing.
It is recommended to allocate 30 MND because it has the greatest effect overall on your healing.
PIE #2
If you know a fight you don't need as much mana/PIE. - But it helps.
VIT #3
If you don't have the HP, you don't have the gear/ilvl for that content.
DET #4
Increases average heal strength by 1%, which is nice, but is under-healing by 10hp a concern of yours?
SS #5
Fast spells.
Crit #6
Over-healing bad.
Piety is the only secondary statistic that alleviates the pressures of playing WHM. Everything else is a bonus. Don't sacrifice anything for VIT/MND/PIE, these are core statistics.