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	$37.98	11.216%	$42.2398	$42.24	$42.25	$42.20
Purchase #2	$123.95	11.714%	$138.4695	$138.47	$138.45	$138.50
Purchase #3	$156.88	9.646%	$172.0126	$172.01	$172.00	$172.00
Purchase #4	$99.47	7.196%	$106.6279	$106.63	$106.65	$106.60
Purchase #5	$104.50	4.295%	$108.9883	$108.99	$109.00	$109.00
Purchase #6	$189.59	9.756%	$208.0864	$208.09	$208.10	$208.10
Purchase #7	$12.66	6.684%	$13.5062	$13.51	$13.50	$13.50
7 purchase totals =	$725.03		$789.9307	$789.94	$789.95	$789.90