We report on ground-state calculations based on current-spin density-functional theory for circular parabolic quantum dots. Hund's first rule, well-known from atomic physics, applies not only at zero magnetic field; analogous rules, related to the level occupancy near the Fermi energy, determine the spin configuration of the ground state in finite magnetic field. Consequently, the addition spectrum of the quantum dot exhibits a specific non-monotonous dependence on magnetic field. We discuss our results with focus on recent experiments which may already contain indications of this new feature.