>write Q on the cash you spend
Especially small denominations.
$1s, $5s get circulated to the public immediately as change, get respent, get recirculated etc.
$20s don't usually get recirculated as change. They get deposited in the bank and recirculated from ATMs.
Banks will be more likely to pull a bill that has been "defaced". A cashier at a store will be less likely to notice or care.