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	$17.43	8.299%	$18.8765	$18.88	$18.90	$18.90
Purchase #2	$9.95	13.204%	$11.2638	$11.26	$11.25	$11.30
Purchase #3	$235.41	10.300%	$259.6572	$259.66	$259.65	$259.70
Purchase #4	$91.49	11.717%	$102.2099	$102.21	$102.20	$102.20
Purchase #5	$79.71	13.443%	$90.4254	$90.43	$90.45	$90.40
Purchase #6	$134.77	11.800%	$150.6729	$150.67	$150.65	$150.70
Purchase #7	$71.69	9.604%	$78.5751	$78.58	$78.60	$78.60
7 purchase totals =	$640.45		$711.6808	$711.69	$711.70	$711.80