Final answer:
Diagnostic casts for evaluating an occlusion should be mounted in centric relation, as this is the most reproducible and stable jaw relation for both diagnosis and treatment procedures.
Step-by-step explanation:
To evaluate an existing occlusion, diagnostic casts should be mounted on an articulator in centric relation. Centric relation is the most reproducible jaw relation and provides a stable reference position from which adjustments can be made during the diagnostic and treatment phases. This position is usually used when restoring occlusion as it represents the jaw position where the condyles are in the most superior position in the glenoid fossa, which is considered the ideal position for occlusion.