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	$17.92	3.309%	$18.513	$18.51	$18.50	$18.50
Purchase #2	$227.42	11.486%	$253.5415	$253.54	$253.55	$253.50
Purchase #3	$20.42	10.797%	$22.6247	$22.62	$22.60	$22.60
Purchase #4	$41.93	3.260%	$43.2969	$43.30	$43.30	$43.30
Purchase #5	$50.72	13.314%	$57.4729	$57.47	$57.45	$57.50
Purchase #6	$30.92	10.422%	$34.1425	$34.14	$34.15	$34.10
Purchase #7	$243.36	4.419%	$254.1141	$254.11	$254.10	$254.10
7 purchase totals =	$632.69		$683.7056	$683.69	$683.65	$683.60