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	$90.99	10.113%	$100.1918	$100.19	$100.20	$100.20
Purchase #2	$79.75	3.312%	$82.3913	$82.39	$82.40	$82.40
Purchase #3	$215.91	11.330%	$240.3726	$240.37	$240.35	$240.40
Purchase #4	$113.94	5.220%	$119.8877	$119.89	$119.90	$119.90
Purchase #5	$94.66	5.666%	$100.0234	$100.02	$100.00	$100.00
Purchase #6	$210.82	5.402%	$222.2085	$222.21	$222.20	$222.20
Purchase #7	$61.93	10.344%	$68.336	$68.34	$68.35	$68.30
7 purchase totals =	$868.00		$933.4113	$933.41	$933.40	$933.40