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	$37.77	6.377%	$40.1786	$40.18	$40.20	$40.20
Purchase #2	$137.57	13.766%	$156.5079	$156.51	$156.50	$156.50
Purchase #3	$93.36	7.469%	$100.3331	$100.33	$100.35	$100.30
Purchase #4	$99.66	7.499%	$107.1335	$107.13	$107.15	$107.10
Purchase #5	$59.72	5.314%	$62.8935	$62.89	$62.90	$62.90
Purchase #6	$239.48	3.408%	$247.6415	$247.64	$247.65	$247.60
Purchase #7	$109.87	4.170%	$114.4516	$114.45	$114.45	$114.50
7 purchase totals =	$777.43		$829.1397	$829.13	$829.20	$829.10