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	$75.34	5.759%	$79.6788	$79.68	$79.70	$79.70
Purchase #2	$134.38	3.771%	$139.4475	$139.45	$139.45	$139.40
Purchase #3	$154.64	9.525%	$169.3695	$169.37	$169.35	$169.40
Purchase #4	$72.46	8.108%	$78.3351	$78.34	$78.35	$78.30
Purchase #5	$19.49	6.223%	$20.7029	$20.70	$20.70	$20.70
Purchase #6	$117.78	11.140%	$130.9007	$130.90	$130.90	$130.90
Purchase #7	$43.80	10.332%	$48.3254	$48.33	$48.35	$48.30
7 purchase totals =	$617.89		$666.7599	$666.77	$666.80	$666.70