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	$29.58	3.287%	$30.5523	$30.55	$30.55	$30.60
Purchase #2	$192.36	12.556%	$216.5127	$216.51	$216.50	$216.50
Purchase #3	$190.41	10.407%	$210.226	$210.23	$210.25	$210.20
Purchase #4	$143.54	10.176%	$158.1466	$158.15	$158.15	$158.10
Purchase #5	$31.46	11.843%	$35.1858	$35.19	$35.20	$35.20
Purchase #6	$215.60	3.786%	$223.7626	$223.76	$223.75	$223.80
Purchase #7	$40.44	12.486%	$45.4893	$45.49	$45.50	$45.50
7 purchase totals =	$843.39		$919.8753	$919.88	$919.90	$919.90