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	$130.93	6.332%	$139.2205	$139.22	$139.20	$139.20
Purchase #2	$200.89	11.298%	$223.5866	$223.59	$223.60	$223.60
Purchase #3	$127.43	13.642%	$144.814	$144.81	$144.80	$144.80
Purchase #4	$171.82	6.559%	$183.0897	$183.09	$183.10	$183.10
Purchase #5	$32.91	3.811%	$34.1642	$34.16	$34.15	$34.20
Purchase #6	$21.48	13.840%	$24.4528	$24.45	$24.45	$24.50
Purchase #7	$80.50	13.871%	$91.6662	$91.67	$91.65	$91.70
7 purchase totals =	$765.96		$840.9940	$840.99	$840.95	$841.10