RR your diagram makes some sense, but it seems like the resulting bullet impact of canting the scope to the right would be lower (because your crosshair is closer to level with the bore) and to the right (because the scope is to the right, and the scope should be set so the bullet matches its point of aim at 2 points during its flight). Maybe I'm looking at it wrong (we're assuming the rifle was sighted without cant before this scenario, yes?), but I do know that a projectile cannot 'rise' at any point during its flight, its drop can only be compensated for by the angle of crosshair in respect to a straight line down the bore.

Do you think it is necessary to use a scope leveler to minimize cant errors in regular shooting, or do you think eyeballing is good enough when installing the scope?