What Olympe said.
However, if the idiom is a purely regional one but is not being used by someone who is from that region, it seems jarring. Sort of like Jimmy speaking like Perry.
It matters, too, whether the idiom is used in the narrative or in spoken dialogue. Unless there is something at the beginning of the story that establishes the narrator's voice (sort of like Mary Alice in Desperate Housewives) then best to avoid regional idioms.
This is really much the same issue as used to occur from time to time with respect to "UKisms" in L & C fic.
c.
