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	$131.36	5.428%	$138.4902	$138.49	$138.50	$138.50
Purchase #2	$233.55	9.848%	$256.55	$256.55	$256.55	$256.60
Purchase #3	$48.42	11.307%	$53.8948	$53.89	$53.90	$53.90
Purchase #4	$105.96	4.793%	$111.0387	$111.04	$111.05	$111.00
Purchase #5	$25.62	13.295%	$29.0262	$29.03	$29.05	$29.00
Purchase #6	$98.39	5.324%	$103.6283	$103.63	$103.65	$103.60
Purchase #7	$156.76	3.647%	$162.477	$162.48	$162.50	$162.50
7 purchase totals =	$800.06		$855.1052	$855.11	$855.20	$855.10