We report on electronic transport measurements in rotational square probe configuration in combination with scanning tunneling potentiometry of epitaxial graphene monolayers which were fabricated by polymer-assisted sublimation growth on SiC substrates. The absence of bilayer graphene on the ultralow step edges of below 0.75 nm scrutinized by atomic force microscopy and scanning tunneling microscopy result in a not yet observed resistance isotropy of graphene on 4H- and 6H-SiC(0001) substrates as low as 2%. We combine microscopic electronic properties with nanoscale transport experiments and thereby disentangle the underlying microscopic scattering mechanism to explain the remaining resistance anisotropy. Eventually, this can be entirely attributed to the resistance and the number of substrate steps which induce local scattering. Thereby, our data represent the ultimate limit for resistance isotropy of epitaxial graphene on SiC for the given miscut of the substrate.