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	$36.88	9.811%	$40.4983	$40.50	$40.50	$40.50
Purchase #2	$78.72	10.784%	$87.2092	$87.21	$87.20	$87.20
Purchase #3	$35.33	13.713%	$40.1748	$40.17	$40.15	$40.20
Purchase #4	$186.35	10.658%	$206.2112	$206.21	$206.20	$206.20
Purchase #5	$34.52	7.343%	$37.0548	$37.05	$37.05	$37.10
Purchase #6	$115.36	10.800%	$127.8189	$127.82	$127.80	$127.80
Purchase #7	$96.42	4.636%	$100.89	$100.89	$100.90	$100.90
7 purchase totals =	$583.58		$639.8572	$639.85	$639.80	$639.90