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	$21.44	7.293%	$23.0036	$23.00	$23.00	$23.00
Purchase #2	$122.76	12.105%	$137.6201	$137.62	$137.60	$137.60
Purchase #3	$79.60	12.685%	$89.6973	$89.70	$89.70	$89.70
Purchase #4	$206.93	7.545%	$222.5429	$222.54	$222.55	$222.50
Purchase #5	$250.76	4.124%	$261.1013	$261.10	$261.10	$261.10
Purchase #6	$69.57	10.455%	$76.8435	$76.84	$76.85	$76.80
Purchase #7	$57.71	3.646%	$59.8141	$59.81	$59.80	$59.80
7 purchase totals =	$808.77		$870.6228	$870.61	$870.60	$870.50