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	$18.96	12.389%	$21.309	$21.31	$21.30	$21.30
Purchase #2	$235.92	3.305%	$243.7172	$243.72	$243.70	$243.70
Purchase #3	$27.79	5.427%	$29.2982	$29.30	$29.30	$29.30
Purchase #4	$12.85	4.707%	$13.4548	$13.45	$13.45	$13.50
Purchase #5	$206.47	7.809%	$222.5932	$222.59	$222.60	$222.60
Purchase #6	$42.84	4.125%	$44.6072	$44.61	$44.60	$44.60
Purchase #7	$51.73	9.128%	$56.4519	$56.45	$56.45	$56.50
7 purchase totals =	$596.56		$631.4315	$631.43	$631.40	$631.50