We present a theoretical study of the anisotropy of the spin relaxation and decoherence in typical quantum wells with an arbitrary magnetic field. In such systems, the orientation of the magnetic field relative to the main crystallographic directions is crucial, owing to the lack of spin-rotation symmetry. For typical high mobility samples, relaxation anisotropies in the motional narrowing limit owing to the interplay of Rashba and Dresselhaus spin orbit coupling are calculated. We also include the effect of the cubic-in-momentum terms. Although commonly ignored in literature, the latter were experimentally evidenced by the observation of strong anisotropy in spin decoherence measurements by different experimental groups and has long remained unexplained.