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	$37.91	5.463%	$39.981	$39.98	$40.00	$40.00
Purchase #2	$174.56	12.351%	$196.1199	$196.12	$196.10	$196.10
Purchase #3	$37.62	10.436%	$41.546	$41.55	$41.55	$41.50
Purchase #4	$45.83	6.390%	$48.7585	$48.76	$48.75	$48.80
Purchase #5	$136.83	6.344%	$145.5105	$145.51	$145.50	$145.50
Purchase #6	$234.33	10.217%	$258.2715	$258.27	$258.25	$258.30
Purchase #7	$97.43	12.175%	$109.2921	$109.29	$109.30	$109.30
7 purchase totals =	$764.51		$839.4795	$839.48	$839.45	$839.50