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	$30.50	4.596%	$31.9018	$31.90	$31.90	$31.90
Purchase #2	$140.83	9.384%	$154.0455	$154.05	$154.05	$154.00
Purchase #3	$239.35	8.583%	$259.8934	$259.89	$259.90	$259.90
Purchase #4	$106.96	5.683%	$113.0385	$113.04	$113.05	$113.00
Purchase #5	$49.67	7.532%	$53.4111	$53.41	$53.40	$53.40
Purchase #6	$54.25	10.477%	$59.9338	$59.93	$59.95	$59.90
Purchase #7	$140.46	5.749%	$148.535	$148.54	$148.55	$148.50
7 purchase totals =	$762.02		$820.7591	$820.76	$820.80	$820.60