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	$19.71	6.509%	$20.9929	$20.99	$21.00	$21.00
Purchase #2	$92.52	13.245%	$104.7743	$104.77	$104.75	$104.80
Purchase #3	$41.99	6.744%	$44.8218	$44.82	$44.80	$44.80
Purchase #4	$156.52	10.294%	$172.6322	$172.63	$172.65	$172.60
Purchase #5	$41.58	4.844%	$43.5941	$43.59	$43.60	$43.60
Purchase #6	$13.25	13.662%	$15.0602	$15.06	$15.05	$15.10
Purchase #7	$48.76	5.443%	$51.414	$51.41	$51.40	$51.40
7 purchase totals =	$414.33		$453.2895	$453.27	$453.25	$453.30