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	$19.25	12.111%	$21.5814	$21.58	$21.60	$21.60
Purchase #2	$143.49	10.710%	$158.8578	$158.86	$158.85	$158.90
Purchase #3	$154.43	9.206%	$168.6468	$168.65	$168.65	$168.60
Purchase #4	$164.70	4.318%	$171.8117	$171.81	$171.80	$171.80
Purchase #5	$30.99	12.816%	$34.9617	$34.96	$34.95	$35.00
Purchase #6	$199.49	9.471%	$218.3837	$218.38	$218.40	$218.40
Purchase #7	$202.95	12.333%	$227.9798	$227.98	$228.00	$228.00
7 purchase totals =	$915.30		$1,002.2229	$1,002.22	$1,002.25	$1,002.30