In terms of linguistic semiotics the underlying opposition lies between the following poles: "common property" collocations vs "private property" collocations, the former signaling the intention of the speakers to convey information or message for the purposes of communication, the latter likely signaling that speakers are intent on esthetic impact, artistic effect, rhetoric or wordplay.
Of equal interest for linguosemiotics are phraseological units like "in general," "by definition," "to take into account," "to take care of," "to be ill," "to be late" etc. They are to be used as is, with no change afflicting the registered dictionary form. Their careful reproduction in speech signals correct acceptable literary usage; violation of word order, or a change of articles or prepositions, would signal bad style, erroneous use and even illiteracy. The learner has to be aware of this system-based sign situation.
The ontology of idioms in speech is drastically different from phraseological units. Let me adduce several examples from the Longman Dictionary of English Idioms (1992): "a new broom sweeps clean" (sometimes shortened to a new broom), "dot the i's" and "cross the tees," "have a finger in every pie" (also "with a finger in every pie"). In studying their semiotic properties we should take into account the present-day socio-cultural tendencies in the use of idioms and attitudes to their adequate use. Adequacy in the use of idioms presupposes that speakers match the "meaning" of the idiom and the particular speech situation adapting the conventionally-shaped and registered form to a new linguistic environment. Creative and sophisticated use of idioms will signal that the speaker is well-educated, linguistically cultured, has a rare gift of the feel for the language. Banal and trite uses of idioms in their dictionary form with practically no attention to the requirements of context and situation will unfailingly signal lack of linguistic culture reflective of either inadequate education or inferior social background.
Let me adduce three quotations from fiction in which idioms are used coherently and adequately, the subtle and elegant variations of form conveying a host of esthetic implications that characterize a work of literature in the strict sense of the word 4: