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	$5.94	7.737%	$6.3996	$6.40	$6.40	$6.40
Purchase #2	$183.63	3.533%	$190.1176	$190.12	$190.10	$190.10
Purchase #3	$105.54	5.617%	$111.4682	$111.47	$111.45	$111.50
Purchase #4	$157.81	10.582%	$174.5095	$174.51	$174.50	$174.50
Purchase #5	$97.63	8.195%	$105.6308	$105.63	$105.65	$105.60
Purchase #6	$193.93	8.560%	$210.5304	$210.53	$210.55	$210.50
Purchase #7	$33.94	13.341%	$38.4679	$38.47	$38.45	$38.50
7 purchase totals =	$778.42		$837.1240	$837.13	$837.10	$837.10