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	$189.34	10.325%	$208.8894	$208.89	$208.90	$208.90
Purchase #2	$139.70	5.587%	$147.505	$147.51	$147.50	$147.50
Purchase #3	$139.40	6.828%	$148.9182	$148.92	$148.90	$148.90
Purchase #4	$40.30	3.382%	$41.6629	$41.66	$41.65	$41.70
Purchase #5	$110.28	11.631%	$123.1067	$123.11	$123.10	$123.10
Purchase #6	$156.32	7.317%	$167.7579	$167.76	$167.75	$167.80
Purchase #7	$38.68	8.491%	$41.9643	$41.96	$41.95	$42.00
7 purchase totals =	$814.02		$879.8044	$879.81	$879.75	$879.90