As you point out, merchants might not have much change ... so my BOB contains 5x$50 bills, 5x$20, 10x$10, 5x$5, 25x$1, and a $10 roll of quarters. And to discourage myself from "borrowing" the emergency money, I've sealed the bills into a small envelope -- it's easy to rip open but makes for a decent psychological barrier against raiding the BOB unnecessarily.