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	$91.99	9.118%	$100.3776	$100.38	$100.40	$100.40
Purchase #2	$75.60	6.657%	$80.6327	$80.63	$80.65	$80.60
Purchase #3	$35.40	13.456%	$40.1634	$40.16	$40.15	$40.20
Purchase #4	$99.31	5.227%	$104.5009	$104.50	$104.50	$104.50
Purchase #5	$51.70	3.408%	$53.4619	$53.46	$53.45	$53.50
Purchase #6	$228.27	3.853%	$237.0652	$237.07	$237.05	$237.10
Purchase #7	$45.50	11.712%	$50.829	$50.83	$50.85	$50.80
7 purchase totals =	$627.77		$667.0307	$667.03	$667.05	$667.10