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	$18.93	6.594%	$20.1782	$20.18	$20.20	$20.20
Purchase #2	$240.38	11.400%	$267.7833	$267.78	$267.80	$267.80
Purchase #3	$98.91	4.715%	$103.5736	$103.57	$103.55	$103.60
Purchase #4	$56.69	5.604%	$59.8669	$59.87	$59.85	$59.90
Purchase #5	$235.81	13.604%	$267.8896	$267.89	$267.90	$267.90
Purchase #6	$173.98	13.637%	$197.7057	$197.71	$197.70	$197.70
Purchase #7	$52.75	13.191%	$59.7083	$59.71	$59.70	$59.70
7 purchase totals =	$877.45		$976.7056	$976.71	$976.70	$976.80