Once you move to storing your prices in pence/cents etc., and roundoff through division will be decimal pence/cents and it's usually ok to round these amounts up or down to the nearest whole pence/cents in favour of the customer without dramatically affecting your bottom line.

The caveat is that you should accumulate all your sums and perform your discounting/exchange rate calculations on the total, not on each individual value prior to totalling.

For example, if your customer is buying 10,000 widgets @ 10 pence each and is entitled to a 17.5% discount.

$unitPrice = 10.0;
$discount = 0.175;
$units = 10_000;

$totalPayable = int( $unitPrice * ( 1 - $discount ) * $units ); ## Cor
+rect way
printf "Total: £%.2f\n", $totalPayable / 100;

##Total: £825.00

$totalPayable = int( $unitPrice * ( 1 - $discount ) ) * $units; ## WRO
+NG!
printf "Total: £%.2f\n", $totalPayable / 100;
## Total: £800.00
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I once sat through a 3 hr long lecture on (UK) accounting conventions related to a very similar problem, the upshot of which was that you should always total up before applying any conversion that required division. The rules probably vary dependant upon where you live or pay taxes.