  Conventionally, one adopts typical momentum flow of a physical observable as
the renormalization scale for its perturbative QCD (pQCD) approximant. This
simple treatment leads to renormalization scheme-and-scale ambiguities due to
the renormalization scheme and scale dependence of the strong coupling and the
perturbative coefficients do not exactly cancel at any fixed order. It is
believed that those ambiguities will be softened by including more higher-order
terms. In the paper, to show how the renormalization scheme dependence changes
when more loop terms have been included, we discuss the sensitivity of pQCD
prediction on the scheme parameters by using the scheme-dependent $\{\beta_{m
\geq 2}\}$-terms. We adopt two four-loop examples, $e^+ e^- \to {\rm hadrons}$
and $\tau$ decays into hadrons, for detailed analysis. Our results show that
under the conventional scale setting, by including more-and-more loop terms,
the scheme dependence of the pQCD prediction cannot be reduced as efficiently
as that of the scale dependence. Thus a proper scale-setting approach should be
important to reduce the scheme dependence. We observe that the principle of
minimum sensitivity could be such a scale-setting approach, which provides a
practical way to achieve optimal scheme and scale by requiring the pQCD
approximate be independent to the "unphysical" theoretical conventions.
