Should sources of sample sentences be shown in (computational) linguistics research? What would be best practice in indicating the source of text samples used to illustrate some concept or analysis result in linguistics or computational linguistics?

For example, I might want to use the sentence 


  If you had told someone in 2012 that in just two years hence, the eurozone would remain bonded together but the United Kingdom might not, they would have thought you insane.


from a recent The New York Times article (http://www.nytimes.com/2014/09/10/upshot/why-does-scotland-want-independence-its-culture-vs-economics.html?abt=0002&amp;abg=0), augmented with some annotation, in order to illustrate how different anaphora resolution methods would work or fail for this real life example. Copyright fair use policies allow to use such limited extracts without any permission, and using them is common practice, but the attribution and quoting seems unclear.

How, and if, should I quote the source of that sentence within a paper?

Would publication style (say, short paper vs. a dissertation) affect it?

Would density of examples (2 examples in whole paper vs 20 different examples in a single page) matter?

Would indirect sources change it? E.g. if the sentences are taken from, say, British National Corpus - but, naturally, they originally come from some different publication.

Language utterance sources are different from other references

At least in computational linguistics, general practice clearly is to not include the source of language utterance or sentence examples together with normal references. There are various approaches seen in practice among respectable publications: no referencing at all, a footnote reference for the sentence, a mention in article text, a single reference to a whole corpus (by referencing not the source but some authoritative paper about the corpus), etc.  I'm wondering about those options, which would be preferable and which should be avoided.

Avoiding the whole problem by using sentences made up by myself is not a good solution - artificial examples are biased towards using constructions that behave in the same way as you and your tools expect, so real life examples of natural language from domain-representative sources are strictly preferred.
I believe your field is doing itself a disservice by not citing the sources of language utterances. Particularly if direct quotes from published sources are used, the information about the source is important. Did the sentence come from the Los Angeles Times or the neighborhood community newspaper? From a novel written by a Novel Prize winner or a first-time novelist? From a book written in 1879 or 1979?

It feels like an archaeologist writing about a pot without including where it was found, what was found around it, etc. 

The field is just not practicing good science. 

If you feel that your paper will not be accepted with more complete referencing of the language utterances, try to mitigate the issue by using examples from the fewest references possible or creating your own with the same structure or pattern.

I think you've found a nice hole in the field--can research be done to show that omitting or not studying the sources of the language utterances changes results?