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	$21.39	8.197%	$23.1433	$23.14	$23.15	$23.10
Purchase #2	$131.41	12.480%	$147.81	$147.81	$147.80	$147.80
Purchase #3	$6.70	6.510%	$7.1362	$7.14	$7.15	$7.10
Purchase #4	$177.98	11.540%	$198.5189	$198.52	$198.50	$198.50
Purchase #5	$157.65	6.570%	$168.0076	$168.01	$168.00	$168.00
Purchase #6	$41.29	10.388%	$45.5792	$45.58	$45.60	$45.60
Purchase #7	$43.54	12.304%	$48.8972	$48.90	$48.90	$48.90
7 purchase totals =	$579.96		$639.0924	$639.10	$639.10	$639.00