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	$24.30	13.118%	$27.4877	$27.49	$27.50	$27.50
Purchase #2	$177.72	9.586%	$194.7562	$194.76	$194.75	$194.80
Purchase #3	$219.90	11.168%	$244.4584	$244.46	$244.45	$244.50
Purchase #4	$129.34	12.289%	$145.2346	$145.23	$145.25	$145.20
Purchase #5	$142.26	6.445%	$151.4287	$151.43	$151.45	$151.40
Purchase #6	$107.66	3.508%	$111.4367	$111.44	$111.45	$111.40
Purchase #7	$35.92	9.204%	$39.2261	$39.23	$39.25	$39.20
7 purchase totals =	$837.10		$914.0284	$914.04	$914.10	$914.00