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	$16.89	10.687%	$18.695	$18.70	$18.70	$18.70
Purchase #2	$123.88	3.716%	$128.4834	$128.48	$128.50	$128.50
Purchase #3	$105.55	5.207%	$111.046	$111.05	$111.05	$111.00
Purchase #4	$45.43	6.286%	$48.2857	$48.29	$48.30	$48.30
Purchase #5	$184.41	12.541%	$207.5369	$207.54	$207.55	$207.50
Purchase #6	$89.36	11.405%	$99.5515	$99.55	$99.55	$99.60
Purchase #7	$27.27	7.482%	$29.3103	$29.31	$29.30	$29.30
7 purchase totals =	$592.79		$642.9088	$642.92	$642.95	$642.90