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	$24.50	11.295%	$27.2673	$27.27	$27.25	$27.30
Purchase #2	$54.52	3.195%	$56.2619	$56.26	$56.25	$56.30
Purchase #3	$83.61	3.437%	$86.4837	$86.48	$86.50	$86.50
Purchase #4	$33.50	8.760%	$36.4346	$36.43	$36.45	$36.40
Purchase #5	$134.66	6.391%	$143.2661	$143.27	$143.25	$143.30
Purchase #6	$131.57	11.507%	$146.7098	$146.71	$146.70	$146.70
Purchase #7	$131.77	8.447%	$142.9006	$142.90	$142.90	$142.90
7 purchase totals =	$594.13		$639.3240	$639.32	$639.30	$639.40