I heard somewhere that the prices ending in 99 cents/pence were introduced to make sure the till was opened to give the customer the penny change. Otherwise, if the customer gave the correct money, whether in dollars or pounds, the cashier could just pocket the money without there being any record of the sale.

In fact, when I see something priced at £1 it somehow looks like a bargain, whereas 99p doesn't (as 99 is a bigger number).