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	$18.35	13.287%	$20.7882	$20.79	$20.80	$20.80
Purchase #2	$127.71	12.622%	$143.8296	$143.83	$143.85	$143.80
Purchase #3	$115.34	11.327%	$128.4046	$128.40	$128.40	$128.40
Purchase #4	$99.53	8.619%	$108.1085	$108.11	$108.10	$108.10
Purchase #5	$48.60	5.467%	$51.257	$51.26	$51.25	$51.30
Purchase #6	$19.53	6.430%	$20.7858	$20.79	$20.80	$20.80
Purchase #7	$160.28	7.315%	$172.0045	$172.00	$172.00	$172.00
7 purchase totals =	$589.34		$645.1782	$645.18	$645.20	$645.20