  A systematic method is proposed for analyzing the renormalization scheme
uncertainties in the next-next-to-leading order QCD predicitions, based on a
condition which eliminates schemes that give rise to large cancellations in the
expression for the characteristic scheme invariant combination of the expansion
coefficients. Using this method it is shown that the QCD corrections to the tau
lepton decay are rather stable with respect to change of the scheme, provided
that an improved formula is used, which involves numerical evaluation of the
contour integral in the complex energy plane with the Adler function under the
integral. Optimized predictions for the tau decay corrections are given. It is
shown that also in the case of the of QCD corrections to $e^{+}e^{-}$
annihilation into hadrons the conventional expansion has sizable scheme
dependence, even at large energies. However, a considerable improvement is
obtained when the QCD corrections are expressed as a contour integral, with the
Adler function under the integral, resumming in this way the large $\pi^{2}$
contributions. In the case of the corrections to the Bjorken sum rule for
polarized structure functions it is found that for $n_{f}=4$ they are
insensitive to change of the scheme. However, the $n_{f}=3$ expression is found
to be strongly scheme dependent at lower energies.
