As a fellow Australian the pennies aren't what I found most bizarre about US and Canadian currency, I more often found myself getting tripped up by the fact that listed prices in US and Canadian stores never include sales tax.
When Canada introduced its federal Goods and Services Tax in the early 1990s, the government wanted to have it included in advertised prices; but under Canada's constitution the regulation of business advertising is a matter of provincial jurisdiction, and the provinces didn't want to play ball.
Most stores in the US don't include sales tax in the listed price, but some do. Examples include vending machines, Starbucks and other coffee shops (but not usually locations inside book stores and supermarkets), and concession stands at concerts, sporting events, and fairs. They probably do it because they don't want to deal with pennies. Concession stands often go further and use whole dollar amounts, since the prices are high anyway.
IIRC, some post office vending machines will take pennies, because they can sell stamps one at a time.
When I was in grad school there was a pizza-by-the-slice place where all prices were tax-included and an exact multiple of 25c. The only coins in their register were quarters. If you gave them any other coin, they counted it out and dumped it in a jar next to the register. A manager claimed that it was way more efficient and easy to count that way (and throughput mattered a lot during busy times). It also meant that a large percentage of the time the customers had exact change counted out before they even got to the register, which meant they didn't even need to make change and throughput increased further.
At least in the US, sales tax is assessed locally, so it can vary even within a particular region. Dealing with fixed prices and additional sales tax is the only reasonable way to run a business that crosses multiple localities.
What about gas prices? The displayed price is what you actually pay, it always includes all the taxes.
I guess the businesses are not required to do this for other products and naturally they choose to display the lower price, i.e. the one excluding taxes.
Yeah, for whatever reason most (all?) states have decided to specifically require that advertised gasoline prices include all taxes and fees, but haven't done the same for other products. Though there are some moves afoot to require it for mobile telephone contracts as well.
