Effect of zero order hold on gain and phase margin in continuous analysis.
One rule of thumb is that if the sample rate is > 10x the highest frequency of interest then a continuous analysis is a reasonable approximation.  However you can check this more accurately.

The frequency response of a ZOH can be estimated from the Laplace transform of its impulse response: Lzoh{s} = (1-e^-sT)/s, and you can substitute s=jw=j*2*pi*f to calculate the frequency response.  With a sampler in front, with no aliasing (band limited input) the net result is L{s}=(1-e^-sT)/(sT).

You can simulate this in Matlab or Octave.  The result is that at one tenth the sample rate (f=fs/10), the gain is down only 0.15dB, but the phase is shifted -18 degrees.  That is quite a bit of phase distortion.  These effects need to be taken into account in any signal processing chain, so the end result is what you intended.