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	$36.63	7.537%	$39.3908	$39.39	$39.40	$39.40
Purchase #2	$121.56	12.625%	$136.907	$136.91	$136.90	$136.90
Purchase #3	$11.59	11.352%	$12.9057	$12.91	$12.90	$12.90
Purchase #4	$59.73	9.647%	$65.4922	$65.49	$65.50	$65.50
Purchase #5	$145.54	6.600%	$155.1456	$155.15	$155.15	$155.10
Purchase #6	$153.72	4.150%	$160.0994	$160.10	$160.10	$160.10
Purchase #7	$212.66	3.342%	$219.7671	$219.77	$219.75	$219.80
7 purchase totals =	$741.43		$789.7078	$789.72	$789.70	$789.70