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	$18.91	8.676%	$20.5506	$20.55	$20.55	$20.60
Purchase #2	$50.48	5.871%	$53.4437	$53.44	$53.45	$53.40
Purchase #3	$111.87	11.128%	$124.3189	$124.32	$124.30	$124.30
Purchase #4	$102.44	7.522%	$110.1455	$110.15	$110.15	$110.10
Purchase #5	$71.52	12.225%	$80.2633	$80.26	$80.25	$80.30
Purchase #6	$189.65	3.583%	$196.4452	$196.45	$196.45	$196.40
Purchase #7	$188.98	3.102%	$194.8422	$194.84	$194.85	$194.80
7 purchase totals =	$733.85		$780.0094	$780.01	$780.00	$779.90