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.
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.
Let time I visit the USA, many shop don't have changes for $100 bills. I found $20 bills the easiest to use.
(I am not an American, was on a work trip)