I've recently posted an answer with prices in Euros, which was promptly edited to include the official euro symbol. So what is the official policy on currency signs?
The problem with using the symbols is that they can be a bit hard (read annoying) to find. My standard North American keyboard comes with $ by default and I don't know where to find Euro, Yen and others without looking them up and copy-pasting: too much work.
Using simple English descriptors is vague, dollar: American ? Canadian ? Australian ? New Zealand ? Peso: Mexican ? Argentinian ? Cuban ? Chilean ?
Writing the ISO codes on the other hand is faster and more legible (in my opinion), and I think a lot of us already know a lot of them by heart from doing basic conversions (I'm guessing ?). And as you mentioned, they're easy to copy paste into search engines (or even spotlight on OSX) to get conversions.
I'm not one for strict rules or even saying we all "should" do something, this is merely my preference and just a suggestion
This question asks for a multi-part answer: When writing, when editing and the general typesetting.
When writing a post you should use whichever format is clear enough and easy enough for you to write. That may (or may not) include the currency symbol if it is present on your keyboard or you know how to generate it. That may include three letter codes such as EUR. That may include plain text forms such as US dollars. All of these should be equally valid in my opinion.
Eur is a non-standard format (as are EURO and others), and should thus not be used.
Ideally, your post should look typographically ‘clean’; however, less ‘clean’ posts should not prevent people from upvoting (and vice-versa).
An edit should typically include more than simply changing one variant to another variant. I admit I am guilty of suggesting a too minor edit here (which was rejected). However, the second edit was equally minor and should have been rejected, too. (Note that the second edit introduced the format symbol-value which, at least for Germany, is less common than value-symbol. Unfortunately I could not find definite sources which types are common/preferred in Germany, let alone in Austria and Slovakia.)
When changing other things within the text, performing a clean-up to adhere to typographic ideals can be warranted, as it is for capitalisation (e.g. I).
From a typesetting perspective
From a typesetting point of view, I would definitely prefer the corresponding currency symbols as long as it is clear which currency is meant. Using € for Euro is unambiguous. Using $ is generally ambiguous, it could be Australian, US, Canada, New Zealand, Fiji … However, if the post is clearly about getting from (say) Atlanta, Georgia to Washington DC, then it is unambiguous enough that US-dollars are meant.
Using three-letter codes can be preferable in ambiguous cases, such as when going from the US to Canada. Using the written words should be restricted to cases where the number is also written out (‘a dollar or two’).
Whichever format you use for writing the unit, the same format should be used for the number. This makes things that belong together look like they belong together. Examples:
It is usually preferable to use simple plain text rather than code markup, since code markup disrupts one’s flow of reading. However, that effect may be desired in given circumstances — if it is used, both number and unit should be formatted using monospace.
In English texts, the three-letter code should preceed the currency value as should the currency symbol. The three-letter code should be separated by a no-break space, which can be either directly entered or entered as
. (A normal space often does no harm but can cause line breaks between currency and value which should be avoided.) According to typical English language style guides, the currency symbol should not be separated by a space. Examples:
In most other European languages, the currency typically follows the value. For example, the following variants are preferred in German:
From a typesetting perspective, both styles are in-line with at least one standard. As long as there is no mixing between the two styles, an edit to convert the continental one into the English one should be avoided. Again, when editing anyway, there is no harm done in harmonising to the English standard.
: Format here is taken to mean plain text,
monospace, MathJax, etc. (although the latter is not active on travel.SE).
: And Irish, Latvian and Maltese, among the languages of the EU. Probably in many other languages outside Europe, too.
: Compare the style guide of the European Union, English version.
: Compare the style guide of the European Union, German version.