I'm still trying to figure this one out also. I think it might have to do with the size of the show. At one show (fairly small) I hardly needed any change, yet at another (medium sized show) I was scrambling to get change and there were no ATM's nearby. You may need some $20's also. This weekend I will be at a craft show where they have had 20,000 people attend in past years. I don't take credit cards yet, so I'm thinking I will need quite a bit more change.
By the way, I don't like to take time out at the show to figure sales tax, so I have a formula that I use with my excel sales tracking spread sheet that figures the sales tax for me. As I'm making my price tags I can just include the sales tax in the price (and then I round up to the nearest $5 or $10 mark). When I get home from the show I plug in the price of the sold piece and it figures out the sales tax for me to report to the Dept. of Revenue. I don't have to worry about keeping coins for change, and it saves so much time at the show, which I can use to talk to customers!
Good luck at your show!!!! Alice