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	$11.71	12.714%	$13.1988	$13.20	$13.20	$13.20
Purchase #2	$126.57	4.865%	$132.7276	$132.73	$132.75	$132.70
Purchase #3	$92.44	6.840%	$98.7629	$98.76	$98.75	$98.80
Purchase #4	$95.89	13.320%	$108.6625	$108.66	$108.65	$108.70
Purchase #5	$136.72	8.853%	$148.8238	$148.82	$148.80	$148.80
Purchase #6	$61.61	10.223%	$67.9084	$67.91	$67.90	$67.90
Purchase #7	$65.61	5.265%	$69.0644	$69.06	$69.05	$69.10
7 purchase totals =	$590.55		$639.1484	$639.14	$639.10	$639.20