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	$5.73	13.656%	$6.5125	$6.51	$6.50	$6.50
Purchase #2	$93.57	11.227%	$104.0751	$104.08	$104.10	$104.10
Purchase #3	$123.89	5.420%	$130.6048	$130.60	$130.60	$130.60
Purchase #4	$127.93	11.813%	$143.0424	$143.04	$143.05	$143.00
Purchase #5	$153.67	7.549%	$165.2705	$165.27	$165.25	$165.30
Purchase #6	$69.27	3.720%	$71.8468	$71.85	$71.85	$71.80
Purchase #7	$142.41	7.681%	$153.3485	$153.35	$153.35	$153.30
7 purchase totals =	$716.47		$774.7006	$774.70	$774.70	$774.60