The axiogingival line angle is the ideal area for placing a retentive groove in crown preparation because it provides maximum resistance and retention by engaging two surfaces simultaneously (axial and gingival). This anatomical landmark allows the groove to be hidden beneath the crown margin, improving esthetics and preventing crown displacement. Additionally, grooves placed here increase the surface area without weakening the tooth structure, enhancing the mechanical interlocking of the crown. Proper placement along the axiogingival line angle is crucial for long-term stability of the restoration.

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