The isomath package makes it possible to typeset upright Greek letters via \mathrm and \mathbf. Is it it possible to force all Greek letters to be typeset upright unless specified otherwise, e.g. to ...
On pg. 144 of Kopka and Daily's A Guide to LaTeX, they mention that the symbols $e$, $i$, $d$, and $\pi$ should be displayed upright in math mode (for their usual uses). This is easy to do for $e$, ...