I never carry cash. It can get lost, I just stick to plastic, one in my wallet and one in a card holder always in my trouser pocket. Besides, no need to keep receipts if you pay with a card, there's a record of every transaction.
So you're out and the interac/cc system goes down...you're fucked. Can't get gas, can't eat. I've seen it happen in this area for a good 4-hour period on two occasions in the past couple of years. I'll never be at the mercy of the phone/computer networks.
My kids can smell when I have cash, so I usually have about $1-$2, along with a debit and credit card. Maybe a couple quarters for the vending machine if I'm lucky.