  Imaging surveys will find many tens to hundreds of thousands of Type Ia
supernovae in the next decade, and measure their light curves. In addition to a
need for characterizing their types and subtypes, a redshift is required to
place them on a Hubble diagram to map the cosmological expansion. We
investigate the requirements on redshift systematics control in order not to
bias cosmological results, in particular dark energy parameter estimation. We
find that additive and multiplicative systematics must be constrained at the
few$\,\times 10^{-3}$ level, effectively requiring spectroscopic followup for
robust use of photometric supernovae. Catastrophic outliers need control at the
subpercent level. We also investigate sculpting the spectroscopic sample.
