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	$75.92	5.536%	$80.1229	$80.12	$80.10	$80.10
Purchase #2	$122.51	4.787%	$128.3746	$128.37	$128.35	$128.40
Purchase #3	$103.62	7.690%	$111.5884	$111.59	$111.60	$111.60
Purchase #4	$31.39	6.139%	$33.317	$33.32	$33.30	$33.30
Purchase #5	$120.46	10.576%	$133.1998	$133.20	$133.20	$133.20
Purchase #6	$103.63	4.140%	$107.9203	$107.92	$107.90	$107.90
Purchase #7	$63.74	12.166%	$71.4946	$71.49	$71.50	$71.50
7 purchase totals =	$621.27		$666.0176	$666.01	$665.95	$666.00