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	$37.78	12.531%	$42.5142	$42.51	$42.50	$42.50
Purchase #2	$117.60	8.341%	$127.409	$127.41	$127.40	$127.40
Purchase #3	$21.51	13.586%	$24.4323	$24.43	$24.45	$24.40
Purchase #4	$142.44	13.453%	$161.6025	$161.60	$161.60	$161.60
Purchase #5	$211.97	6.571%	$225.8985	$225.90	$225.90	$225.90
Purchase #6	$45.93	12.283%	$51.5716	$51.57	$51.55	$51.60
Purchase #7	$6.77	3.484%	$7.0059	$7.01	$7.00	$7.00
7 purchase totals =	$584.00		$640.4340	$640.43	$640.40	$640.40