Idioms in Writing
It happens, those terms we use in colloquial language slip into our writing. Idioms have a meaning not necessarily gathered from the words used.

Frequency of Idioms in speech
One of my mother’s favourite, when we wanted to get a job done quickly, was “Rome wasn’t built in a day.” It meant we should be patient and take our time. And of course, a city such as Rome would have taken years to build, but that wasn’t the point.
“Mend your fences” was another I heard in my childhood when I wasn’t familiar with the term idiom. I soon learned that it meant to make peace with someone who might have given me a hard time, or we might have given each other trouble. “Off the top of your head” means “without careful thought or investigation” according to the Oxford Dictionary of Idioms.*
A term might be noted as a regionalism, when it’s often repeated in a specific area of the country and not known in another region. How that term developed is anyone’s guess.

Idioms slip into our writing too
As an editor, I often come across this kind of terminology in writers’ work. I ask, “Will your readers understand the meaning?”
If you’re not there to share the intended meaning, or the meaning is not clear in the context, you risk the reader misunderstanding your message.
Such terms can be confusing for not only native English speakers who don’t recognize the term, but also those learning English as a second language. For example, “make hay” means to make good use of the opportunity while it lasts, while “hit the hay” means “go to bed.” Someone learning the language may consult a dictionary and interpret it literally. That’s where confusion enters.

Who are your readers?  
Who’s in your audience? When in doubt, leave out the idiom, unless a character in your book speaks that way. And if you’re going to use the idiom, be sure to build in a way to illustrate the meaning.
 When in doubt, leave the idiom out.
 
*Oxford Dictionary of Idioms, 2004, Oxford University Press