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	$19.58	6.608%	$20.8738	$20.87	$20.85	$20.90
Purchase #2	$135.83	9.122%	$148.2204	$148.22	$148.20	$148.20
Purchase #3	$233.35	9.217%	$254.8579	$254.86	$254.85	$254.90
Purchase #4	$113.29	13.417%	$128.4901	$128.49	$128.50	$128.50
Purchase #5	$193.59	3.482%	$200.3308	$200.33	$200.35	$200.30
Purchase #6	$49.68	4.704%	$52.0169	$52.02	$52.00	$52.00
Purchase #7	$21.33	6.493%	$22.715	$22.72	$22.70	$22.70
7 purchase totals =	$766.65		$827.5049	$827.51	$827.45	$827.50