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	$81.58	10.313%	$89.9933	$89.99	$90.00	$90.00
Purchase #2	$83.43	5.674%	$88.1638	$88.16	$88.15	$88.20
Purchase #3	$57.94	10.232%	$63.8684	$63.87	$63.85	$63.90
Purchase #4	$165.96	3.740%	$172.1669	$172.17	$172.15	$172.20
Purchase #5	$33.48	6.367%	$35.6117	$35.61	$35.60	$35.60
Purchase #6	$80.36	6.517%	$85.5971	$85.60	$85.60	$85.60
Purchase #7	$100.56	5.665%	$106.2567	$106.26	$106.25	$106.30
7 purchase totals =	$603.31		$641.6579	$641.66	$641.60	$641.80