  Evaluating parton density systematic uncertainties in Monte~Carlo event
generator predictions has long been achieved by reweighting between the
original and systematic PDFs for the initial state configurations of the
individual simulated events. This weighting is now pre-emptively performed in
many generators, providing convenient weight factors for PDF and scale
systematics -- including for NLO calculations where counterterms make the
weight calculation complex. This note attempts a pedagogical discussion and
empirical study of the consequences of neglecting the effects of PDF variations
on the beyond-fixed-order components of MC models, and the implications for
parton shower & MPI tuning strategies. We confirm that the effects are usually
small, for well-understood reasons, and discuss the connected issue of
consistent treatment of the strong coupling between PDFs and parton showers,
where motivations from physical principles and the need for good
data-description are not always well-aligned.
