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	$12.39	9.612%	$13.5809	$13.58	$13.60	$13.60
Purchase #2	$146.40	8.526%	$158.8821	$158.88	$158.90	$158.90
Purchase #3	$183.46	13.493%	$208.2143	$208.21	$208.20	$208.20
Purchase #4	$171.30	11.806%	$191.5237	$191.52	$191.50	$191.50
Purchase #5	$99.90	8.392%	$108.2836	$108.28	$108.30	$108.30
Purchase #6	$237.36	3.738%	$246.2325	$246.23	$246.25	$246.20
Purchase #7	$50.34	9.551%	$55.148	$55.15	$55.15	$55.10
7 purchase totals =	$901.15		$981.8651	$981.85	$981.90	$981.80