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	$38.93	3.337%	$40.2291	$40.23	$40.25	$40.20
Purchase #2	$128.71	9.439%	$140.8589	$140.86	$140.85	$140.90
Purchase #3	$180.81	5.366%	$190.5123	$190.51	$190.50	$190.50
Purchase #4	$131.30	10.697%	$145.3452	$145.35	$145.35	$145.30
Purchase #5	$16.85	6.802%	$17.9961	$18.00	$18.00	$18.00
Purchase #6	$166.89	4.783%	$174.8723	$174.87	$174.85	$174.90
Purchase #7	$130.33	8.146%	$140.9467	$140.95	$140.95	$140.90
7 purchase totals =	$793.82		$850.7606	$850.77	$850.75	$850.70