In the article, Out Of The Ashes, (New Era, May 2004), San Diego teens discuss their experience surviving and then cleaning up the California wildfires of 2003. They include several preparedness tips that they gained from their experiences. Their first tip is to keep some cash on hand. I think this tip is applicable and important in almost every emergency situation. I similarly recommend that you keep a small portion of your financial reserve as "cash on hand."
How much? Again, this is very personal. I try to keep a small amount in several locations such as in the car, in each 72-hour kit, in my purse, and some in my home. We also keep a jar of change. All this cash adds up. I already find that I turn to that cash (and then have to replace it) in minor emergencies. In major emergencies, banks might be closed, ATMs run out of money, and if electricity is out, stores can't take credit cards.
It's also important to have your cash reserve in small bills. You may remember in a post about Hurricane entitled, Prepared In Houston, that the author was forced to pay $5 extra because the stores couldn't make change for her. So even $5 bills may be too big. I recommend that you keep $1 bills and/or change on hand.