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	$18.58	10.152%	$20.4662	$20.47	$20.45	$20.50
Purchase #2	$129.49	7.511%	$139.216	$139.22	$139.20	$139.20
Purchase #3	$123.96	11.524%	$138.2452	$138.25	$138.25	$138.20
Purchase #4	$19.75	6.387%	$21.0114	$21.01	$21.00	$21.00
Purchase #5	$22.29	6.353%	$23.7061	$23.71	$23.70	$23.70
Purchase #6	$138.59	4.498%	$144.8238	$144.82	$144.80	$144.80
Purchase #7	$137.82	10.519%	$152.3173	$152.32	$152.30	$152.30
7 purchase totals =	$590.48		$639.7860	$639.80	$639.70	$639.70