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	$36.79	6.397%	$39.1435	$39.14	$39.15	$39.10
Purchase #2	$93.67	10.510%	$103.5147	$103.51	$103.50	$103.50
Purchase #3	$211.50	10.842%	$234.4308	$234.43	$234.45	$234.40
Purchase #4	$54.69	9.635%	$59.9594	$59.96	$59.95	$60.00
Purchase #5	$97.95	5.301%	$103.1423	$103.14	$103.15	$103.10
Purchase #6	$12.48	8.177%	$13.5005	$13.50	$13.50	$13.50
Purchase #7	$110.82	7.839%	$119.5072	$119.51	$119.50	$119.50
7 purchase totals =	$617.90		$673.1984	$673.19	$673.20	$673.10