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	$19.26	4.667%	$20.1589	$20.16	$20.15	$20.20
Purchase #2	$123.34	7.685%	$132.8187	$132.82	$132.80	$132.80
Purchase #3	$115.40	4.689%	$120.8111	$120.81	$120.80	$120.80
Purchase #4	$50.83	10.222%	$56.0258	$56.03	$56.05	$56.00
Purchase #5	$29.59	7.676%	$31.8613	$31.86	$31.85	$31.90
Purchase #6	$144.78	12.250%	$162.5156	$162.52	$162.50	$162.50
Purchase #7	$56.65	10.522%	$62.6107	$62.61	$62.60	$62.60
7 purchase totals =	$539.85		$586.8021	$586.81	$586.75	$586.80