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	$18.89	11.414%	$21.0461	$21.05	$21.05	$21.00
Purchase #2	$119.74	13.441%	$135.8343	$135.83	$135.85	$135.80
Purchase #3	$91.30	12.265%	$102.4979	$102.50	$102.50	$102.50
Purchase #4	$105.79	5.547%	$111.6582	$111.66	$111.65	$111.70
Purchase #5	$95.66	10.461%	$105.667	$105.67	$105.65	$105.70
Purchase #6	$80.25	10.620%	$88.7726	$88.77	$88.75	$88.80
Purchase #7	$160.68	7.279%	$172.3759	$172.38	$172.40	$172.40
7 purchase totals =	$672.31		$737.8520	$737.86	$737.85	$737.90