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	$27.60	8.104%	$29.8367	$29.84	$29.85	$29.80
Purchase #2	$240.71	10.766%	$266.6248	$266.62	$266.60	$266.60
Purchase #3	$57.91	4.182%	$60.3318	$60.33	$60.35	$60.30
Purchase #4	$94.83	10.571%	$104.8545	$104.85	$104.85	$104.90
Purchase #5	$55.54	7.360%	$59.6277	$59.63	$59.65	$59.60
Purchase #6	$58.99	12.356%	$66.2788	$66.28	$66.30	$66.30
Purchase #7	$11.45	13.539%	$13.0002	$13.00	$13.00	$13.00
7 purchase totals =	$547.03		$600.5545	$600.55	$600.60	$600.50