At stores I usually pay in exact change when possible. I figure that puts off (minutely) the time until the cashier has to refill the trays and decreases the number of trips the merchant has to make to the bank for coins and small bills.
I realize, though, that the cashier can give me change faster than it takes for me to dig out my own change and so I’m slowing down the transaction. Would it actually be more to the merchant’s benefit for me to just hand over a $20 and move along?