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	$35.51	12.687%	$40.0152	$40.02	$40.00	$40.00
Purchase #2	$136.29	9.285%	$148.9445	$148.94	$148.95	$148.90
Purchase #3	$178.67	6.467%	$190.2246	$190.22	$190.20	$190.20
Purchase #4	$88.57	12.394%	$99.5474	$99.55	$99.55	$99.50
Purchase #5	$56.69	8.510%	$61.5143	$61.51	$61.50	$61.50
Purchase #6	$18.77	13.493%	$21.3026	$21.30	$21.30	$21.30
Purchase #7	$136.27	6.193%	$144.7092	$144.71	$144.70	$144.70
7 purchase totals =	$650.77		$706.2578	$706.25	$706.20	$706.10