Only when a defined uncertainty cannot be resolved any other way.
Use demonstrations, focused workshops, configuration walkthroughs and supporting evidence first. A POC may be justified for an unusual assessment method, complex hierarchy and roll-up logic, a material data-volume concern or critical integration behaviour. Test that uncertainty rather than attempting to recreate your complete ERM environment.
If procurement requires one, confirm a senior sponsor, committed participants, representative data and success criteria before asking the supplier to commit, and agree the supplier's effort and any commercial arrangements up front. If these are not ready, use a focused workshop to clarify the requirement first.