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	$19.82	6.782%	$21.1642	$21.16	$21.15	$21.20
Purchase #2	$80.55	11.613%	$89.9043	$89.90	$89.90	$89.90
Purchase #3	$231.61	11.662%	$258.6204	$258.62	$258.60	$258.60
Purchase #4	$78.71	9.334%	$86.0568	$86.06	$86.05	$86.10
Purchase #5	$45.80	4.215%	$47.7305	$47.73	$47.75	$47.70
Purchase #6	$210.66	13.192%	$238.4503	$238.45	$238.45	$238.50
Purchase #7	$183.99	3.688%	$190.7756	$190.78	$190.80	$190.80
7 purchase totals =	$851.14		$932.7021	$932.70	$932.70	$932.80