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	$139.99	4.614%	$146.4491	$146.45	$146.45	$146.40
Purchase #2	$137.52	9.298%	$150.3066	$150.31	$150.30	$150.30
Purchase #3	$25.25	13.260%	$28.5982	$28.60	$28.60	$28.60
Purchase #4	$40.25	6.278%	$42.7769	$42.78	$42.80	$42.80
Purchase #5	$34.61	12.336%	$38.8795	$38.88	$38.90	$38.90
Purchase #6	$63.78	10.448%	$70.4437	$70.44	$70.45	$70.40
Purchase #7	$56.62	9.830%	$62.1857	$62.19	$62.20	$62.20
7 purchase totals =	$498.02		$539.6397	$539.65	$539.70	$539.60