The cavosurface margin of an inlay preparation should be bevelled to increase the surface area for the restorative material to bond effectively. This improves the marginal seal, reduces microleakage, and enhances the mechanical retention of the inlay. Additionally, beveling helps in distributing occlusal forces more evenly, which minimizes the risk of fracture at the margin. A bevel also facilitates a smoother transition between the tooth and restoration, improving esthetic outcomes and longevity of the restoration.

Reference: Principles and Practice of Operative Dentistry, Clifford M. Sturdevant, 5th Edition.