  We evaluate both the tau lepton energy loss produced by photonuclear
interactions and the neutrino charged current cross section at ultra-high
energies, relevant to neutrino bounds with Earth-skimming tau neutrinos, using
different theoretical and phenomenological models for nucleon and nucleus
structure functions. The theoretical uncertainty is estimated by taking
different extrapolations of the structure function F2 to very low values of x,
in the low and moderate Q2 range for the tau lepton interaction and at high Q2
for the neutrino-nucleus inelastic cross section. It is at these extremely low
values of x where nuclear shadowing and parton saturation effects are unknown
and could be stronger than usually considered. For tau and neutrino energies
E=10^9 GeV we find uncertainties of a factor 4 for the tau energy loss and of a
factor 2 for the charged current neutrino-nucleus cross section.
