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	$38.41	11.387%	$42.7837	$42.78	$42.80	$42.80
Purchase #2	$128.54	8.825%	$139.8837	$139.88	$139.90	$139.90
Purchase #3	$207.86	3.547%	$215.2328	$215.23	$215.25	$215.20
Purchase #4	$55.82	8.328%	$60.4687	$60.47	$60.45	$60.50
Purchase #5	$76.34	9.857%	$83.8648	$83.86	$83.85	$83.90
Purchase #6	$48.25	7.204%	$51.7259	$51.73	$51.75	$51.70
Purchase #7	$187.31	6.284%	$199.0806	$199.08	$199.10	$199.10
7 purchase totals =	$742.53		$793.0402	$793.03	$793.10	$793.10