When I asked my wife, whom is native Japanese born & bred, this question, she first remarked that when growing up, her parents never even refrigerated their expensive miso pastes (in the summertime no less) and they never got sick and the miso never went bad (it's fermented and salt laden after all). In our household, my wife says refrigerated miso can last you at least a year - it simply starts drying out when it gets old.
Update: My wife now says her parents used to even mix in the mold which might grow on their miso, thereby enhancing the taste.
Aside: My parents used to tell me the fine soys (soy sauce) made in the old days had a crust of mold, maggots, etc during the fermentation process during manufacture.