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	$7.42	10.645%	$8.2099	$8.21	$8.20	$8.20
Purchase #2	$68.61	3.625%	$71.0971	$71.10	$71.10	$71.10
Purchase #3	$102.94	8.671%	$111.8659	$111.87	$111.85	$111.90
Purchase #4	$47.78	5.541%	$50.4275	$50.43	$50.45	$50.40
Purchase #5	$152.97	7.139%	$163.8905	$163.89	$163.90	$163.90
Purchase #6	$213.86	11.394%	$238.2272	$238.23	$238.25	$238.20
Purchase #7	$180.75	8.471%	$196.0613	$196.06	$196.05	$196.10
7 purchase totals =	$774.33		$839.7794	$839.79	$839.80	$839.80