In a Class V cavity preparation, the axial wall is designed to be flat when viewed mesiodistally to ensure proper adaptation of restorative material and to maintain structural integrity. A flat axial wall provides a uniform thickness of restorative material, preventing stress concentration and reducing the risk of fracture. It also helps in achieving a better marginal seal and retention form. Unlike other cavity classes, the Class V cavity does not require concave or convex axial walls as it is primarily located on the smooth surfaces of the crown. Maintaining a flat axial wall simplifies cavity preparation and enhances restoration longevity.

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