The axioincisal angle is the most common site for placing a retentive groove because it provides an ideal location to enhance mechanical retention in cavity preparations, especially in Class III and IV restorations. This groove helps prevent restoration displacement by engaging the restorative material within a defined undercut area. Placing the groove here utilizes the natural anatomic form, preserving tooth structure while increasing resistance to dislodgement during function. Other locations like the occlusal fossa or pulpal floor are generally avoided to prevent weakening the tooth or compromising the pulp. The cervical margin is primarily for seal, not retention.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.