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	$5.69	13.847%	$6.4779	$6.48	$6.50	$6.50
Purchase #2	$54.40	10.664%	$60.2012	$60.20	$60.20	$60.20
Purchase #3	$128.75	10.379%	$142.113	$142.11	$142.10	$142.10
Purchase #4	$72.74	10.628%	$80.4708	$80.47	$80.45	$80.50
Purchase #5	$132.37	8.868%	$144.1086	$144.11	$144.10	$144.10
Purchase #6	$96.48	7.237%	$103.4623	$103.46	$103.45	$103.50
Purchase #7	$240.55	12.256%	$270.0318	$270.03	$270.05	$270.00
7 purchase totals =	$730.98		$806.8656	$806.86	$806.85	$806.90