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	$7.35	11.871%	$8.2225	$8.22	$8.20	$8.20
Purchase #2	$127.82	9.228%	$139.6152	$139.62	$139.60	$139.60
Purchase #3	$113.82	12.165%	$127.6662	$127.67	$127.65	$127.70
Purchase #4	$165.55	11.738%	$184.9823	$184.98	$185.00	$185.00
Purchase #5	$44.25	12.702%	$49.8706	$49.87	$49.85	$49.90
Purchase #6	$10.25	12.872%	$11.5694	$11.57	$11.55	$11.60
Purchase #7	$59.90	3.160%	$61.7928	$61.79	$61.80	$61.80
7 purchase totals =	$528.94		$583.7190	$583.72	$583.65	$583.80