The rest seat on a canine should be V-shaped to properly engage the cingulum of the canine tooth, which provides optimal mechanical retention and stability for the removable partial denture. The V-shape conforms to the tooth anatomy, directing occlusal forces along the long axis of the tooth, reducing torque and preventing tooth damage. This shape also allows for ease of insertion and removal of the prosthesis while maintaining adequate support. A U-shaped or flat rest would fail to harness the natural contour of the tooth, compromising retention and increasing stress. Therefore, the V-shaped rest seat ensures effective load distribution and prosthesis function.

Reference: Stewart's Clinical Removable Partial Prosthodontics, Stewart, 4th Edition.