  Photonic crystal waveguides are known to support C-points - point-like
polarisation singularities with local chirality. Such points can couple with
dipole-like emitters to produce highly directional emission, from which
spin-photon entanglers can be built. Much is made of the promise of using
slow-light modes to enhance this light-matter coupling. Here we explore the
transition from travelling to standing waves for two different photonic crystal
waveguide designs. We find that time-reversal symmetry and the reciprocal
nature of light places constraints on using C-points in the slow-light regime.
We observe two distinctly different mechanisms through which this condition is
satisfied in the two waveguides. In the waveguide designs we consider, a modest
group-velocity of $v_g \approx c/10$ is found to be the optimum for slow-light
coupling to the C-points.
