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	$170.50	12.136%	$191.1919	$191.19	$191.20	$191.20
Purchase #2	$186.37	4.123%	$194.054	$194.05	$194.05	$194.10
Purchase #3	$79.43	12.712%	$89.5271	$89.53	$89.55	$89.50
Purchase #4	$102.76	6.221%	$109.1527	$109.15	$109.15	$109.20
Purchase #5	$52.84	5.426%	$55.7071	$55.71	$55.70	$55.70
Purchase #6	$145.93	8.165%	$157.8452	$157.85	$157.85	$157.80
Purchase #7	$40.74	5.241%	$42.8752	$42.88	$42.90	$42.90
7 purchase totals =	$778.57		$840.3532	$840.36	$840.40	$840.40