This paper addresses the problem of assessing to which extent a bidding zone delineation requires redispatching actions after the clearing of energy markets. Such issue arises in particular when policy makers review the organisation of electricity markets and question the design of uniform price zones. To avoid relying on arbitrarily-defined sensitive parameters, we propose a metrics based on information that is sufficient to perform nodal pricing for the whole region under consideration. The metrics is illustrated with a realistic model of the West-European power system considering two application cases, namely i) selecting a clustering technique to define candidate bidding zones, and ii) selecting candidate splitting and merging options for pre-existing bidding zones. Results show that the proposed index provides a consistent order of magnitude of the redispatching needs subsequent to market clearing, although it requires no specific user-defined parameters.