  (Abridged) The X-ray background can be used to constrain the accretion
history of Supermassive Black Holes (SMBHs) in Active Galactic Nuclei (AGN). A
knowledge of the hard X-ray bolometric correction, \kappa_{2-10keV} is a vital
input into these studies. Variations in the disk emission in the UV have not
previously been taken into account in calculating \kappa_{2-10keV}; we show
that such variations are important by constructing optical--to--X-ray SEDs for
54 AGN. In particular, we use FUSE UV and X-ray data from the literature to
constrain the disk emission as well as possible. Previous work has suggested a
dependence of \kappa_{2-10keV} on AGN luminosity, but we find significant
spread in \kappa_{2-10keV} with no simple dependence on luminosity. Populations
such as Narrow-Line Seyfert 1 nuclei (NLS1s), Radio Loud and X-ray Weak AGN may
have values of \kappa_{2-10keV} differing systematically from the rest of the
AGN population. Other sources of uncertainty include intrinsic extinction in
the optical--UV, X-ray and UV variability and uncertainties in SMBH mass
estimates. Our results suggest a more well-defined relationship between
\kappa_{2-10keV} and Eddington ratio in AGN, with a transitional region at an
Eddington ratio of ~0.1, below which the bolometric correction is typically 15
- 25, and above which it is typically 40 - 70. We consider the potential
implied parallels with the low/hard and high/soft states in Galactic Black Hole
(GBH) accretion, and present bolometric corrections for the GBH binary GX 339-4
for comparison. Our findings reinforce previous studies proposing a multi-state
description of AGN accretion analogous to that for GBH binaries. Future
calculations of the SMBH mass density may need to account for the possible
dependence of \kappa_{2-10keV} on Eddington ratio.
