Which one is preferred to write x-axis as a single unit that cannot be hyphenated?

Question

Which one is preferred to write x-axis as a single unit that cannot be hyphenated by LaTeX?

\documentclass[preview,varwidth]{standalone}
\usepackage{amsmath}
\begin{document}
\preview
Along the $x\text{-axis}$,
\endpreview

\preview
Along the \mbox{$x$-axis},
\endpreview
\end{document}

As a completeness, I don't think

Along the $x$-axis,

is recommended because hyphenation may sometimes occur.

Answer 1

As far as I know, the options are equivalent output-wise, so the question is about a semantic choice.

In my opinion then, the answer is the same as for all compound words containing math elements : they are words you use in an english sentence, not in a math formula. So the latter option would make more sense.

And since I'm lazy, I wouldn't bother writing \mbox{...} all the time, I would just add it in the final version of the document where needed.

Answer 2

Let me note that there is not any typographical rule that states that the below method is any way correct. It can merely be used if one wishes to control the dash's placement.

One way to do this is to add a command which adds a non-hyphenated dash.

This can be done with \nobreakdash. However, this will not allow any hyphenation until the next word (i.e. no hyphenation in the axis word). Thus one should add a space which breaks the words

A solution would be

% Requires amsmath package
\def\dirdash{\nobreakdash-\hspace{0pt}}

This will show this:

\documentclass{article}
\usepackage{geometry,amsmath}
\geometry{paperheight=5cm,paperwidth=6cm,textheight=3cm,textwidth=4cm}
\def\dirdash#1{\nobreakdash#1\hspace{0pt}}
\begin{document}
Hello, this is the a $x$-axis

Hello, this is the a $x$\dirdash-axis

Hello, this is the a $x$-longword

Hello, this is the a $x$\dirdash-longword
\end{document}

And yield the following output