Money will still have value to the optimists, the people who think society will recover and is in a rough patch. Even if not accepted nearly everywhere, they will hoard it (and likely break into banks and collect more).
Some warlords or community organizers might insist on continuing to use currency within their borders, and pseudo-government continuations or Army bases might as well, although this is all enforced economies at the point of a gun.
And time will decide the value of currencies. A day later, a week later, a year later. All different situations and it depends on how "overrun" everything is. If it's just Shaun of the Dead, and you have a bad couple of days and then everyone adjusts and returns to semi-normalcy, then money will always have value. In the Dawn of the Dead (2004) situation, which covers a few months, the economy has crashed, but may recover if people adjust.