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	$18.64	13.230%	$21.1061	$21.11	$21.10	$21.10
Purchase #2	$113.85	9.705%	$124.8991	$124.90	$124.90	$124.90
Purchase #3	$92.61	8.246%	$100.2466	$100.25	$100.25	$100.20
Purchase #4	$99.35	6.431%	$105.7392	$105.74	$105.75	$105.70
Purchase #5	$88.45	11.570%	$98.6837	$98.68	$98.70	$98.70
Purchase #6	$121.91	7.625%	$131.2056	$131.21	$131.20	$131.20
Purchase #7	$107.43	3.720%	$111.4264	$111.43	$111.45	$111.40
7 purchase totals =	$642.24		$693.3067	$693.32	$693.35	$693.20