  We investigate the possibility of spin-preserving symmetries due to the
interplay of Rashba and Dresselhaus spin-orbit coupling in n-doped zinc-blende
semiconductor quantum wells of general crystal orientation. It is shown that a
conserved spin operator can be realized if and only if at least two
growth-direction Miller indices agree in modulus. The according spin-orbit
field has in general both in-plane and out-of-plane components and is always
perpendicular to the shift vector of the corresponding persistent spin helix.
We also analyze higher-order effects arising from the Dresselhaus term, and the
impact of our results on weak (anti)localization corrections.
