  The CMB's B-mode polarization provides a handle on several cosmological
parameters most notably the tensor-to-scalar ratio, $r$, and is sensitive to
parameters which govern the growth of large scale structure (LSS) and evolution
of the gravitational potential. The primordial gravitational-wave- and
secondary lensing-induced B-mode signals are very weak and therefore prone to
various foregrounds and systematics. In this work we use Fisher-matrix-based
estimations and apply, for the first time, Monte-Carlo Markov Chain (MCMC)
simulations to determine the effect of beam systematics on the inferred
cosmological parameters from five upcoming experiments: PLANCK, POLARBEAR,
SPIDER, QUIET+CLOVER and CMBPOL. We consider beam systematics which couple the
beam substructure to the gradient of temperature anisotropy and polarization
(differential beamwidth, pointing and ellipticity) and beam systematics due to
differential beam normalization (differential gain) and orientation (beam
rotation) of the polarization-sensitive axes (the latter two effects are
insensitive to the beam substructure). We determine allowable levels of beam
systematics for given tolerances on the induced parameter errors and check for
possible biases in the inferred parameters concomitant with potential increases
in the statistical uncertainty. All our results are scaled to the 'worst case
scenario'. In this case and for our tolerance levels, the beam rotation should
not exceed the few-degree to sub-degree level, typical ellipticity is required
to be 1% level, the differential gain allowed level is a few parts in $10^{3}$
to $10^{4}$, differential beamwidth upper limits are of the sub-percent level
and differential pointing should not exceed the few- to sub-arcsec level.
