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	$138.58	9.172%	$151.2906	$151.29	$151.30	$151.30
Purchase #2	$129.47	10.168%	$142.6345	$142.63	$142.65	$142.60
Purchase #3	$142.70	11.190%	$158.6681	$158.67	$158.65	$158.70
Purchase #4	$119.37	5.776%	$126.2648	$126.26	$126.25	$126.30
Purchase #5	$90.77	4.121%	$94.5106	$94.51	$94.50	$94.50
Purchase #6	$38.34	9.722%	$42.0674	$42.07	$42.05	$42.10
Purchase #7	$110.55	12.646%	$124.5302	$124.53	$124.55	$124.50
7 purchase totals =	$769.78		$839.9662	$839.96	$839.95	$840.00