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	$34.94	11.759%	$39.0486	$39.05	$39.05	$39.00
Purchase #2	$33.36	11.325%	$37.138	$37.14	$37.15	$37.10
Purchase #3	$38.38	11.343%	$42.7334	$42.73	$42.75	$42.70
Purchase #4	$226.60	7.395%	$243.3571	$243.36	$243.35	$243.40
Purchase #5	$135.59	11.267%	$150.8669	$150.87	$150.85	$150.90
Purchase #6	$68.45	7.532%	$73.6057	$73.61	$73.60	$73.60
Purchase #7	$80.92	10.126%	$89.114	$89.11	$89.10	$89.10
7 purchase totals =	$618.24		$675.8637	$675.87	$675.85	$675.80