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	$249.34	12.423%	$280.3155	$280.32	$280.30	$280.30
Purchase #2	$145.67	8.402%	$157.9092	$157.91	$157.90	$157.90
Purchase #3	$36.85	7.406%	$39.5791	$39.58	$39.60	$39.60
Purchase #4	$198.40	5.284%	$208.8835	$208.88	$208.90	$208.90
Purchase #5	$211.93	4.866%	$222.2425	$222.24	$222.25	$222.20
Purchase #6	$166.41	8.787%	$181.0324	$181.03	$181.05	$181.00
Purchase #7	$34.89	11.346%	$38.8486	$38.85	$38.85	$38.80
7 purchase totals =	$1,043.49		$1,128.8108	$1,128.81	$1,128.85	$1,128.70