We present detailed low-temperature scanning tunneling spectroscopy (STS) investigations of the local electronic density of states (DOS) on the fivefold and twofold i-Al70Pd21Mn9; pseudo tenfold ξ′-Al72Pd25Mn3; tenfold d-Al77Ni17Co6; twofold-(12110) d-Al72.9Ni10.4Co16.7; (100) Y-Al75.8Ni2.1Co22.1; and (111) Al surface in the range of +/− 1 eV around the Fermi energy. All these quasicrystals and approximants exhibit the theoretically predicted electronic pseudogap. We observe no evidence for a correlation between the depth of this pseudogap, as measured by STS, and the remarkably high electrical resistivity of the investigated quasicrystals and approximants. A parametrization of the spatial variability of STS spectra is introduced as a measure of the magnitude of the electron localization for a given system. The resulting Ŝ parameter is found to be proportional to the square root of the electrical resistivity. This finding supports the validity of interpreting spatial variations of fine-structure features in the STS spectra of Al-based quasicrystals and approximants in terms of electron localization.