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	$81.50	10.635%	$90.1675	$90.17	$90.15	$90.20
Purchase #2	$130.86	5.325%	$137.8283	$137.83	$137.85	$137.80
Purchase #3	$225.93	8.663%	$245.5023	$245.50	$245.50	$245.50
Purchase #4	$212.32	13.111%	$240.1573	$240.16	$240.15	$240.20
Purchase #5	$147.76	6.471%	$157.3215	$157.32	$157.30	$157.30
Purchase #6	$52.68	12.506%	$59.2682	$59.27	$59.25	$59.30
Purchase #7	$45.62	8.277%	$49.396	$49.40	$49.40	$49.40
7 purchase totals =	$896.67		$979.6411	$979.65	$979.60	$979.70