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	$224.90	6.656%	$239.8693	$239.87	$239.85	$239.90
Purchase #2	$91.64	5.501%	$96.6811	$96.68	$96.70	$96.70
Purchase #3	$69.96	9.284%	$76.4551	$76.46	$76.45	$76.50
Purchase #4	$120.50	9.229%	$131.6209	$131.62	$131.60	$131.60
Purchase #5	$5.45	13.411%	$6.1809	$6.18	$6.20	$6.20
Purchase #6	$66.68	9.300%	$72.8812	$72.88	$72.90	$72.90
Purchase #7	$35.64	12.669%	$40.1552	$40.16	$40.15	$40.20
7 purchase totals =	$614.77		$663.8437	$663.85	$663.85	$664.00