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	$27.55	9.874%	$30.2703	$30.27	$30.25	$30.30
Purchase #2	$90.64	3.570%	$93.8758	$93.88	$93.90	$93.90
Purchase #3	$74.83	4.745%	$78.3807	$78.38	$78.40	$78.40
Purchase #4	$110.36	5.846%	$116.8116	$116.81	$116.80	$116.80
Purchase #5	$9.68	4.824%	$10.147	$10.15	$10.15	$10.10
Purchase #6	$139.43	5.199%	$146.679	$146.68	$146.70	$146.70
Purchase #7	$98.76	6.536%	$105.215	$105.22	$105.20	$105.20
7 purchase totals =	$551.25		$581.3794	$581.39	$581.40	$581.40