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	$37.83	4.315%	$39.4624	$39.46	$39.45	$39.50
Purchase #2	$51.86	9.742%	$56.9122	$56.91	$56.90	$56.90
Purchase #3	$119.48	13.758%	$135.9181	$135.92	$135.90	$135.90
Purchase #4	$60.77	12.462%	$68.3432	$68.34	$68.35	$68.30
Purchase #5	$113.30	13.175%	$128.2273	$128.23	$128.25	$128.20
Purchase #6	$98.28	13.257%	$111.309	$111.31	$111.30	$111.30
Purchase #7	$62.50	6.632%	$66.645	$66.65	$66.65	$66.60
7 purchase totals =	$544.02		$606.8172	$606.82	$606.80	$606.70