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	$19.88	5.785%	$21.0301	$21.03	$21.05	$21.00
Purchase #2	$95.65	13.713%	$108.7665	$108.77	$108.75	$108.80
Purchase #3	$178.38	6.777%	$190.4688	$190.47	$190.45	$190.50
Purchase #4	$142.54	13.138%	$161.2669	$161.27	$161.25	$161.30
Purchase #5	$100.81	3.130%	$103.9654	$103.97	$103.95	$104.00
Purchase #6	$58.52	4.696%	$61.2681	$61.27	$61.25	$61.30
Purchase #7	$139.70	8.393%	$151.425	$151.43	$151.45	$151.40
7 purchase totals =	$735.48		$798.1908	$798.21	$798.15	$798.30