The importance of parallel magnetic field perturbations in gyrokinetic simulations of electromagnetic instabilities and turbulence at mid-radius in the burning plasma phase of the conceptual high-β, reactor-scale, tight-aspect-ratio tokamak STEP is discussed. Previous studies have revealed the presence of unstable hybrid kinetic ballooning modes (KBMs) and subdominant microtearing modes (MTMs) at binormal scales approaching the ion-Larmor radius. Crucially, it was found that the hybrid kinetic ballooning mode requires the inclusion of parallel magnetic field perturbations for instability. Here, the extent to which the inclusion of parallel magnetic field perturbations can be relaxed is explored through gyrokinetic simulations. In particular, the frequently used MHD-approximation (setting the ∇B drift frequency equal to the curvature drift frequency) is discussed and simulations explore whether this approximation is useful for modelling STEP plasmas. If it were valid for STEP, the MHD-approximation would facilitate higher fidelity analysis using present day tools and models. It is shown that the one implementation of the MHD-approximation can reproduce some of the linear properties of the full STEP gyrokinetic system, but nonlinear simulations using the MHD-approximation result in a very different transport states. Unstable modes with very long binormal wavelengths (which are stable when the MHD-approximation is used) are identified as being responsible for this difference.