I have always had between $200 and $1,000 in cash at home and another $40-50 in the car, just for emergencies.
A habit I acquired from my mother. When she died, I knew it would be a long, tedious chore to discover all her hiding places (pockets, cuffs and hems of clothing, etc.). I'm pretty sure we found it all, and it worked out to almost $2,000 worth.
That was just in the one room she lived in at the Alzheimers facility where she died. For the last ten years of her life, she was asking me for cash on a regular basis, but very rarely spent more than a few bucks at a time. I knew she was just squirreling it away, but so what? A child of the Great Depression, she did what made sense to her.
I also keep at least $100 worth of foreign currency (Canadian dollars, British pounds, Euros, Icelandic kronur, and Brazilian reais). I've generally found that ATMs in airports have hefty fees and give you a lousy exchange rate, so I want some cash to get into the city and use a "real" ATM at a bank.