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	$34.86	12.829%	$39.3322	$39.33	$39.35	$39.30
Purchase #2	$211.56	5.795%	$223.8199	$223.82	$223.80	$223.80
Purchase #3	$238.87	4.642%	$249.9583	$249.96	$249.95	$250.00
Purchase #4	$76.71	11.461%	$85.5017	$85.50	$85.50	$85.50
Purchase #5	$122.38	11.179%	$136.0609	$136.06	$136.05	$136.10
Purchase #6	$155.42	7.306%	$166.775	$166.78	$166.80	$166.80
Purchase #7	$165.70	3.709%	$171.8458	$171.85	$171.85	$171.80
7 purchase totals =	$1,005.50		$1,073.2938	$1,073.30	$1,073.30	$1,073.30