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	$19.55	7.550%	$21.026	$21.03	$21.05	$21.00
Purchase #2	$61.66	11.353%	$68.6603	$68.66	$68.65	$68.70
Purchase #3	$228.50	11.449%	$254.661	$254.66	$254.65	$254.70
Purchase #4	$217.72	13.645%	$247.4279	$247.43	$247.45	$247.40
Purchase #5	$144.93	4.840%	$151.9446	$151.94	$151.95	$151.90
Purchase #6	$166.41	8.299%	$180.2204	$180.22	$180.20	$180.20
Purchase #7	$51.86	11.659%	$57.9064	$57.91	$57.90	$57.90
7 purchase totals =	$890.63		$981.8466	$981.85	$981.85	$981.80