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	$18.83	13.197%	$21.315	$21.32	$21.30	$21.30
Purchase #2	$147.53	11.626%	$164.6818	$164.68	$164.70	$164.70
Purchase #3	$130.33	11.349%	$145.1212	$145.12	$145.10	$145.10
Purchase #4	$200.27	7.758%	$215.8069	$215.81	$215.80	$215.80
Purchase #5	$6.51	5.348%	$6.8582	$6.86	$6.85	$6.90
Purchase #6	$33.40	13.462%	$37.8963	$37.90	$37.90	$37.90
Purchase #7	$15.95	9.636%	$17.4869	$17.49	$17.50	$17.50
7 purchase totals =	$552.82		$609.1663	$609.18	$609.15	$609.20