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	$19.98	6.490%	$21.2767	$21.28	$21.30	$21.30
Purchase #2	$155.80	10.705%	$172.4784	$172.48	$172.50	$172.50
Purchase #3	$191.67	9.569%	$210.0109	$210.01	$210.00	$210.00
Purchase #4	$136.91	4.269%	$142.7547	$142.75	$142.75	$142.80
Purchase #5	$102.42	10.254%	$112.9221	$112.92	$112.90	$112.90
Purchase #6	$105.97	6.828%	$113.2056	$113.21	$113.20	$113.20
Purchase #7	$143.89	4.456%	$150.3017	$150.30	$150.30	$150.30
7 purchase totals =	$856.64		$922.9501	$922.95	$922.95	$923.00