  We consider current observational constraints on the presence of cool,
optically thick disk material in quiescent black hole binaries, specifically
focusing on a case study of the prototypical system A0620-00. Such material
might be expected to be present theoretically, but is usually claimed to make a
negligible contribution at optical and infrared wavelengths. The primary
argument is based on measurements of the veiling of stellar photospheric
absorption lines, in which it is assumed that the disk spectrum is featureless.
We use simulated spectra to explore the sensitivity of veiling measurements to
uncertainties in companion temperature, gravity, and metallicity. We find that
the derived veiling is extremely sensitive to a mismatch between the
temperature and metallicity of the companion and template, but that the effect
of a plausible gravity mismatch is much smaller. In general the resulting
uncertainty in the amount of veiling is likely to be much larger than the
usually quoted statistical uncertainty. We also simulate spectra in which the
disk has an emergent spectrum similar to the star and find that in this case,
optical veiling constraints are moderately robust. This is because the
rotational broadening of the disk is so large that the two line profiles
effectively decouple and the measurement of the depth of stellar lines is
largely unbiased by the disk component. We note, however, that this is only
true at intermediate resolutions or higher, and that significant bias might
still affect low resolution IR observations. [Abridged]
