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	$18.89	12.729%	$21.2945	$21.29	$21.30	$21.30
Purchase #2	$106.68	7.251%	$114.4154	$114.42	$114.40	$114.40
Purchase #3	$215.72	3.303%	$222.8452	$222.85	$222.85	$222.80
Purchase #4	$9.85	4.599%	$10.303	$10.30	$10.30	$10.30
Purchase #5	$210.84	10.781%	$233.5707	$233.57	$233.55	$233.60
Purchase #6	$41.36	10.701%	$45.7859	$45.79	$45.80	$45.80
Purchase #7	$62.73	12.366%	$70.4872	$70.49	$70.50	$70.50
7 purchase totals =	$666.07		$718.7019	$718.71	$718.70	$718.70