I think it would depend on how much you are charging for each of your items, and whether you have a credit card reader. Generally, I would take about $100. Have a couple of $20s, a few $10s and $5s, but keep more (about $50 of the $100) in $1s and change. You will keep getting the bigger bills from the customers, and using up the little ones, so the more little ones you have the better. You just need the bigger bills for the first few customers.
If you can keep the price of your items in whole dollars, you won't have to keep much change, but then again, studies have shown that pricing things under a whole dollar ($1.99) does entice them to buy more. I'm pretty sure most people have caught on to this strategy and it probably doesn't affect them as much anymore.