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	$109.94	12.346%	$123.5132	$123.51	$123.50	$123.50
Purchase #2	$97.64	6.788%	$104.2678	$104.27	$104.25	$104.30
Purchase #3	$134.89	11.572%	$150.4995	$150.50	$150.50	$150.50
Purchase #4	$87.71	3.609%	$90.8755	$90.88	$90.90	$90.90
Purchase #5	$156.47	12.261%	$175.6548	$175.65	$175.65	$175.70
Purchase #6	$178.27	6.500%	$189.8576	$189.86	$189.85	$189.90
Purchase #7	$13.83	10.377%	$15.2651	$15.27	$15.25	$15.30
7 purchase totals =	$778.75		$849.9335	$849.94	$849.90	$850.10