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	$18.92	13.584%	$21.4901	$21.49	$21.50	$21.50
Purchase #2	$134.67	12.543%	$151.5617	$151.56	$151.55	$151.60
Purchase #3	$122.43	9.125%	$133.6017	$133.60	$133.60	$133.60
Purchase #4	$128.33	7.546%	$138.0138	$138.01	$138.00	$138.00
Purchase #5	$199.81	11.192%	$222.1727	$222.17	$222.15	$222.20
Purchase #6	$173.88	7.830%	$187.4948	$187.49	$187.50	$187.50
Purchase #7	$241.27	11.852%	$269.8653	$269.87	$269.85	$269.90
7 purchase totals =	$1,019.31		$1,124.2001	$1,124.19	$1,124.15	$1,124.30