  Often, the microscopic interaction mechanism of an open quantum system gives
rise to a `counter term' which renormalises the system Hamiltonian. Such term
compensates for the distortion of the system's potential due to the finite
coupling to the environment. Even if the coupling is weak, the counter term is,
in general, not negligible. Similarly, weak-coupling master equations feature a
number of `Lamb-shift terms' which, contrary to popular belief, cannot be
neglected. Yet, the practice of vanishing both counter term and Lamb shift when
dealing with master equations is almost universal; and, surprisingly, it can
yield better results. By accepting the conventional wisdom, one may approximate
the dynamics more accurately and, importantly, the resulting master equation is
guaranteed to equilibrate to the correct steady state in the high-temperature
limit. In this paper we discuss why is this the case. Specifically, we show
that, if the potential distortion is small -- but non-negligible -- the counter
term does not influence any dissipative processes to second order in the
coupling. Furthermore, we show that, for large environmental cutoff, the
Lamb-shift terms approximately cancel any coherent effects due to the counter
term -- this renders the combination of both contributions irrelevant in
practice. We thus provide precise conditions under which the open-system
folklore regarding Lamb shift and counter terms is rigorously justified.
