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	$12.89	4.486%	$13.4682	$13.47	$13.45	$13.50
Purchase #2	$142.73	11.299%	$158.8571	$158.86	$158.85	$158.90
Purchase #3	$141.64	13.543%	$160.8223	$160.82	$160.80	$160.80
Purchase #4	$234.89	3.361%	$242.7847	$242.78	$242.80	$242.80
Purchase #5	$127.64	7.185%	$136.8109	$136.81	$136.80	$136.80
Purchase #6	$154.25	3.638%	$159.8616	$159.86	$159.85	$159.90
Purchase #7	$45.60	6.357%	$48.4988	$48.50	$48.50	$48.50
7 purchase totals =	$859.64		$921.1036	$921.10	$921.05	$921.20