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	$20.34	4.248%	$21.204	$21.20	$21.20	$21.20
Purchase #2	$234.82	8.490%	$254.7562	$254.76	$254.75	$254.80
Purchase #3	$99.34	4.164%	$103.4765	$103.48	$103.50	$103.50
Purchase #4	$193.75	3.310%	$200.1631	$200.16	$200.15	$200.20
Purchase #5	$84.87	5.118%	$89.2136	$89.21	$89.20	$89.20
Purchase #6	$101.56	3.833%	$105.4528	$105.45	$105.45	$105.50
Purchase #7	$74.94	11.488%	$83.5491	$83.55	$83.55	$83.50
7 purchase totals =	$809.62		$857.8153	$857.81	$857.80	$857.90