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	$19.25	11.567%	$21.4766	$21.48	$21.50	$21.50
Purchase #2	$52.47	7.609%	$56.4624	$56.46	$56.45	$56.50
Purchase #3	$143.99	10.468%	$159.0629	$159.06	$159.05	$159.10
Purchase #4	$103.63	4.276%	$108.0612	$108.06	$108.05	$108.10
Purchase #5	$12.45	10.673%	$13.7788	$13.78	$13.80	$13.80
Purchase #6	$78.78	8.474%	$85.4558	$85.46	$85.45	$85.50
Purchase #7	$7.85	6.350%	$8.3485	$8.35	$8.35	$8.30
7 purchase totals =	$418.42		$452.6462	$452.65	$452.65	$452.80