It is important to have cash on hand in an emergency. It needs to be in small bills and kept water proof in a small bag. If you were to go to the store to buy a loaf of bread they would not be able to give you change and you may pay $10 for bread.(if there is anything left on the shelf)
I try to put aside $10 here and there but regret I keep finding other items to buy for my kit instead. I read somewhere that it is a good idea to have about $200 on hand. I would put it in a few different places in case the bulk of it gets lost, destroyed or stolen.
Of course we would still have credit and debit cards, but after a major disaster the machines will no longer be in service. Banks will not be dealing with a crazy rush of people wanting their money out when the computers are all down. If you have a lot of cash just laying around, put a bit in your kit:) If you are like the rest of us you will have to take a little out of each paycheck until your stash adds up. If you don't already have a safe for documents and such, it would also be a good idea to buy one to store your money in. They have some with that are small with handles that would be easy to grab in and emergency.