Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$16.56	8.584%	$17.9815	$17.98	$18.00	$18.00
Purchase #2	$238.67	10.256%	$263.148	$263.15	$263.15	$263.10
Purchase #3	$48.72	8.476%	$52.8495	$52.85	$52.85	$52.80
Purchase #4	$149.59	12.395%	$168.1317	$168.13	$168.15	$168.10
Purchase #5	$38.96	6.835%	$41.6229	$41.62	$41.60	$41.60
Purchase #6	$128.49	3.625%	$133.1478	$133.15	$133.15	$133.10
Purchase #7	$70.37	3.476%	$72.8161	$72.82	$72.80	$72.80
7 purchase totals =	$691.36		$749.6975	$749.70	$749.70	$749.50