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	$35.59	11.171%	$39.5658	$39.57	$39.55	$39.60
Purchase #2	$229.36	11.647%	$256.0736	$256.07	$256.05	$256.10
Purchase #3	$114.54	3.796%	$118.8879	$118.89	$118.90	$118.90
Purchase #4	$46.61	10.244%	$51.3847	$51.38	$51.40	$51.40
Purchase #5	$70.38	12.240%	$78.9945	$78.99	$79.00	$79.00
Purchase #6	$178.30	10.430%	$196.8967	$196.90	$196.90	$196.90
Purchase #7	$81.53	9.504%	$89.2786	$89.28	$89.30	$89.30
7 purchase totals =	$756.31		$831.0818	$831.08	$831.10	$831.20