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	$90.45	10.210%	$99.6849	$99.68	$99.70	$99.70
Purchase #2	$121.37	6.814%	$129.6402	$129.64	$129.65	$129.60
Purchase #3	$21.40	13.425%	$24.273	$24.27	$24.25	$24.30
Purchase #4	$71.74	7.255%	$76.9447	$76.94	$76.95	$76.90
Purchase #5	$85.80	5.545%	$90.5576	$90.56	$90.55	$90.60
Purchase #6	$145.71	4.514%	$152.2873	$152.29	$152.30	$152.30
Purchase #7	$59.30	11.257%	$65.9754	$65.98	$66.00	$66.00
7 purchase totals =	$595.77		$639.3631	$639.36	$639.40	$639.40