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	$18.57	7.702%	$20.0003	$20.00	$20.00	$20.00
Purchase #2	$234.34	13.114%	$265.0713	$265.07	$265.05	$265.10
Purchase #3	$54.53	9.867%	$59.9105	$59.91	$59.90	$59.90
Purchase #4	$81.31	11.759%	$90.8712	$90.87	$90.85	$90.90
Purchase #5	$11.77	11.754%	$13.1534	$13.15	$13.15	$13.20
Purchase #6	$10.88	13.108%	$12.3062	$12.31	$12.30	$12.30
Purchase #7	$164.74	4.485%	$172.1286	$172.13	$172.15	$172.10
7 purchase totals =	$576.14		$633.4415	$633.44	$633.40	$633.50