Ooh, I wanna jump in too!
I've seen plenty of bank ATMs that give cash in increments of $10 and $20. I don't know the ratio, but $10 is common enough that I can get slightly annoyed when I find one that only gives $20s
If memory serves, once I took out an amount in the hundreds, and I got lots of $10 bills. Odd.
Never seen any give less than $10 though.
If we agreed on $50 and cash-only, and you showed up with three $20s, I'd gladly take it. Oh, you want change? Well, I MIGHT have a $5 bill, but that's it; the only other bills I have are $20s (just like you!).