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	$34.90	11.679%	$38.976	$38.98	$39.00	$39.00
Purchase #2	$43.87	4.396%	$45.7985	$45.80	$45.80	$45.80
Purchase #3	$171.45	9.577%	$187.8698	$187.87	$187.85	$187.90
Purchase #4	$140.83	13.669%	$160.0801	$160.08	$160.10	$160.10
Purchase #5	$48.81	9.565%	$53.4787	$53.48	$53.50	$53.50
Purchase #6	$75.52	4.500%	$78.9184	$78.92	$78.90	$78.90
Purchase #7	$75.77	13.223%	$85.7891	$85.79	$85.80	$85.80
7 purchase totals =	$591.15		$650.9106	$650.92	$650.95	$651.00