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	$18.81	13.719%	$21.3905	$21.39	$21.40	$21.40
Purchase #2	$128.45	3.640%	$133.1256	$133.13	$133.15	$133.10
Purchase #3	$133.36	12.754%	$150.3687	$150.37	$150.35	$150.40
Purchase #4	$144.65	4.527%	$151.1983	$151.20	$151.20	$151.20
Purchase #5	$93.69	12.613%	$105.5071	$105.51	$105.50	$105.50
Purchase #6	$179.45	9.227%	$196.0079	$196.01	$196.00	$196.00
Purchase #7	$79.54	5.345%	$83.7914	$83.79	$83.80	$83.80
7 purchase totals =	$777.95		$841.3895	$841.40	$841.40	$841.40