[msbarnett]

"If the customer has the pennies, they can use them. Payments with debit or credit cards, or cheques, can also be to the penny. But if the customer is paying cash and doesn't have the pennies, the total will go up or down to the nearest nickel."

I gather that's post application of sales tax.

[mindslight]

Presuming a uniform distribution of price endings, existing pennies are then worth 1.43 cents (you only spend them when the price is congruent to (0.03 or 0.04) mod 0.05).