The SI conventions state that variables and quantity symbols should be italicized, while unit symbols should be roman (see, e.g., this useful reference/cheat sheet). For example,
Here the proton mass (m_p) is being used as a quantity. But you could use the same quantity as a unit, in which case should you use a roman font? For example,
Or is it really that SI units specifically should be in roman font, and everything else should be italicized?