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	$5.58	10.591%	$6.171	$6.17	$6.15	$6.20
Purchase #2	$157.68	9.527%	$172.7022	$172.70	$172.70	$172.70
Purchase #3	$181.82	3.702%	$188.551	$188.55	$188.55	$188.60
Purchase #4	$12.47	3.139%	$12.8614	$12.86	$12.85	$12.90
Purchase #5	$80.34	13.850%	$91.4671	$91.47	$91.45	$91.50
Purchase #6	$54.49	8.691%	$59.2257	$59.23	$59.25	$59.20
Purchase #7	$108.52	4.468%	$113.3687	$113.37	$113.35	$113.40
7 purchase totals =	$600.90		$644.3471	$644.35	$644.30	$644.50