  We demonstrate the importance of correctly implementing RG-running and
choosing the RG-scale when calculating quantum corrections to inflaton
dynamics. We show that such corrections are negligible for single-field
inflation, in the sense of not altering the viable region in the $n_s-r$ plane,
when imposing Planck constraints on $A_s$. Surprisingly, this also applies, in
a nontrivial way, for an inflaton coupled to additional spectator degrees of
freedom. The result relies on choosing the renormalisation scale
(pseudo-)optimally, thereby avoiding unphysical large logarithmic corrections
to the Friedmann equations and large running of the couplings. We find that the
viable range of parameters of the potential is altered relative to the
classical limit, and we find an upper limit of $g\simeq 10^{-4}$ on the value
of the inflaton-spectator portal coupling still allowing for inflation. And an
upper limit of $g\simeq 10^{-5}$ for inflation to correctly reproduce the
scalar amplitude of fluctuations $A_s$.
