  General Relativistic (GR) Magnetohydrodynamic (MHD) simulations of black hole
accretion find significant magnetic stresses near and inside the innermost
stable circular orbit (ISCO), suggesting that such flows could radiate in a
manner noticeably different from the prediction of the standard model, which
assumes that there are no stresses in that region. We provide estimates of how
phenomenologically interesting parameters like the ``radiation edge", the
innermost ring of the disc from which substantial thermal radiation escapes to
infinity, may be altered by stresses near the ISCO. These estimates are based
on data from a large number of three-dimensional GRMHD simulations combined
with GR ray-tracing. For slowly spinning black holes ($a/M<0.9$), the radiation
edge lies well inside where the standard model predicts, particularly when the
system is viewed at high inclination. For more rapidly spinning black holes,
the contrast is smaller. At fixed total luminosity, the characteristic
temperature of the accretion flow increases between a factor of $1.2-2.4$ over
that predicted by the standard model, whilst at fixed mass accretion rate,
there is a corresponding enhancement of the accretion luminosity which may be
anywhere from tens of percent to order unity. When all these considerations are
combined, we find that, for fixed black hole mass, luminosity, and inclination
angle, our uncertainty in the characteristic temperature of the radiation
reaching distant observers due to uncertainty in dissipation profile (around a
factor of 3) is {\it greater} than the uncertainty due to a complete lack of
knowledge of the black hole's spin (around a factor of 2) and furthermore that
spin estimates based on the stress-free inner boundary condition provide an
upper limit to $a/M$.
