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	$28.83	3.727%	$29.9045	$29.90	$29.90	$29.90
Purchase #2	$129.46	4.633%	$135.4579	$135.46	$135.45	$135.50
Purchase #3	$142.93	8.140%	$154.5645	$154.56	$154.55	$154.60
Purchase #4	$67.25	13.746%	$76.4942	$76.49	$76.50	$76.50
Purchase #5	$88.32	8.236%	$95.594	$95.59	$95.60	$95.60
Purchase #6	$68.94	13.561%	$78.289	$78.29	$78.30	$78.30
Purchase #7	$72.65	5.151%	$76.3922	$76.39	$76.40	$76.40
7 purchase totals =	$598.38		$646.6963	$646.68	$646.70	$646.80