I recommend having a mix of bills; the other person may not have change.
Clarification: Sure, keeping a $100 bill tucked away in the wallet for emergencies is a great idea (and I do that too), but wherever you keep emergency supplies, I suggest having a mix of smaller bills.
Absolutely. The $100's are for getting a hotel room or something. The $20's are likely the largest other merchants will break.
My emergency cash stash has a couple $100's and a ton of $5's because I'm not super limited on space, and when everyone else has $20's and needs them broken, a bunch of $5's makes everything easy, and beats carrying a whole bucket of $1's. Learned that at flea-markets.
Clarification: Sure, keeping a $100 bill tucked away in the wallet for emergencies is a great idea (and I do that too), but wherever you keep emergency supplies, I suggest having a mix of smaller bills.