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	$37.46	5.453%	$39.5027	$39.50	$39.50	$39.50
Purchase #2	$215.67	12.372%	$242.3527	$242.35	$242.35	$242.40
Purchase #3	$51.64	11.477%	$57.5667	$57.57	$57.55	$57.60
Purchase #4	$47.52	10.754%	$52.6303	$52.63	$52.65	$52.60
Purchase #5	$238.74	11.469%	$266.1211	$266.12	$266.10	$266.10
Purchase #6	$89.84	11.326%	$100.0153	$100.02	$100.00	$100.00
Purchase #7	$159.36	3.303%	$164.6237	$164.62	$164.60	$164.60
7 purchase totals =	$840.23		$922.8125	$922.81	$922.75	$922.80