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	$250.84	11.423%	$279.4935	$279.49	$279.50	$279.50
Purchase #2	$135.62	12.858%	$153.058	$153.06	$153.05	$153.10
Purchase #3	$102.49	11.601%	$114.3799	$114.38	$114.40	$114.40
Purchase #4	$119.42	3.430%	$123.5161	$123.52	$123.50	$123.50
Purchase #5	$102.31	5.512%	$107.9493	$107.95	$107.95	$107.90
Purchase #6	$84.53	7.160%	$90.5823	$90.58	$90.60	$90.60
Purchase #7	$19.26	11.249%	$21.4266	$21.43	$21.45	$21.40
7 purchase totals =	$814.47		$890.4057	$890.41	$890.45	$890.40