Nowadays, if you're paying cash... you gotta prepay. That's when it happens.
If I was using a credit card and wanted exactly $20 worth of gas, it's still hard to do... cuz the gas pump flow is the same as before. Yet, the price is really fast. One tiny squeeze and it jumps several cents. Back then when gas was only $1 a gallon, it was easy to give it a little squeeze to get an exact amount.