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	$7.78	8.125%	$8.4121	$8.41	$8.40	$8.40
Purchase #2	$140.96	11.679%	$157.4227	$157.42	$157.40	$157.40
Purchase #3	$103.34	5.122%	$108.6331	$108.63	$108.65	$108.60
Purchase #4	$80.65	10.676%	$89.2602	$89.26	$89.25	$89.30
Purchase #5	$18.49	12.359%	$20.7752	$20.78	$20.80	$20.80
Purchase #6	$80.85	7.379%	$86.8159	$86.82	$86.80	$86.80
Purchase #7	$126.33	4.410%	$131.9012	$131.90	$131.90	$131.90
7 purchase totals =	$558.40		$603.2204	$603.22	$603.20	$603.20