What is the proper way to space between variables?

Question

There are many questions out there already that answer how to add horizontal spacing in math mode and also how to deal with a number in front of a variable.

So my question, with all these different spacings one can add in mathematical equations. Is there any convention in using spacing between variables?

For example, if all my variables were 1 character only (with an optional descriptive subscript) then I could write something like equation 1 and be able to move variables around without there being any confusion as to what the variables are if the reader would know beforehand that all variables are one character only. But if I have variables with multiple characters as well than there might be confusion as in equation 2, where one might think that Re is suddenly a variable of its own like in equation 4 instead of two separate variables. So then how do we deal with this? What's the general way to deal with this? (The general way I deal with it is to add spacing between the variables as in equation 7, but the right hand side looks a bit weird to me with all the spacing, or is equation 6 better?)

Sample code:

\documentclass{article}

\begin{document}

    \begin{equation}
        R = \frac{x}{e}
    \end{equation}

    \begin{equation}
        Re = x
    \end{equation}

    \begin{equation}
        R \, e = x
    \end{equation}

    \begin{equation}
        Re = \frac{\rho v d}{\mu}
    \end{equation}

    \begin{equation}
        Re\mu = \rho v d
    \end{equation}

    \begin{equation}
        Re \, \mu = \rho v d
    \end{equation}

    \begin{equation}
        Re \, \mu = \rho \, v \, d
    \end{equation}

\end{document}