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	$70.98	12.720%	$80.0087	$80.01	$80.00	$80.00
Purchase #2	$142.49	6.216%	$151.3472	$151.35	$151.35	$151.30
Purchase #3	$156.59	13.119%	$177.133	$177.13	$177.15	$177.10
Purchase #4	$69.37	3.523%	$71.8139	$71.81	$71.80	$71.80
Purchase #5	$70.56	4.165%	$73.4988	$73.50	$73.50	$73.50
Purchase #6	$69.79	10.140%	$76.8667	$76.87	$76.85	$76.90
Purchase #7	$146.67	4.163%	$152.7759	$152.78	$152.80	$152.80
7 purchase totals =	$726.45		$783.4442	$783.45	$783.45	$783.40