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	$88.59	12.641%	$99.7887	$99.79	$99.80	$99.80
Purchase #2	$136.29	8.528%	$147.9128	$147.91	$147.90	$147.90
Purchase #3	$53.83	12.318%	$60.4608	$60.46	$60.45	$60.50
Purchase #4	$11.66	13.539%	$13.2386	$13.24	$13.25	$13.20
Purchase #5	$53.99	3.338%	$55.7922	$55.79	$55.80	$55.80
Purchase #6	$124.39	8.119%	$134.4892	$134.49	$134.50	$134.50
Purchase #7	$116.66	9.642%	$127.9084	$127.91	$127.90	$127.90
7 purchase totals =	$585.41		$639.5907	$639.59	$639.60	$639.60