There's a little-known rule, best stated as
“Operators and constants with a fixed meaning should be set upright.”
This results in more readable equations overall, and adds clarity:2
However, I have my doubts as to how far this rule should extend. It is obvious to typeset π, i, or e upright. But should I also typeset the physical constants that I use in my publications upright? Where should one draw the line? Can the simple "little-known rule" mentioned above be extended?