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	$18.94	13.118%	$21.4245	$21.42	$21.40	$21.40
Purchase #2	$90.50	9.264%	$98.8839	$98.88	$98.90	$98.90
Purchase #3	$6.72	4.233%	$7.0045	$7.00	$7.00	$7.00
Purchase #4	$227.51	10.387%	$251.1415	$251.14	$251.15	$251.10
Purchase #5	$80.94	6.118%	$85.8919	$85.89	$85.90	$85.90
Purchase #6	$190.65	7.254%	$204.4798	$204.48	$204.50	$204.50
Purchase #7	$52.56	9.713%	$57.6652	$57.67	$57.65	$57.70
7 purchase totals =	$667.82		$726.4913	$726.48	$726.50	$726.50