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	$6.97	13.191%	$7.8894	$7.89	$7.90	$7.90
Purchase #2	$179.71	7.274%	$192.7821	$192.78	$192.80	$192.80
Purchase #3	$85.78	4.451%	$89.5981	$89.60	$89.60	$89.60
Purchase #4	$225.34	12.867%	$254.3345	$254.33	$254.35	$254.30
Purchase #5	$22.88	4.265%	$23.8558	$23.86	$23.85	$23.90
Purchase #6	$209.49	5.363%	$220.7249	$220.72	$220.70	$220.70
Purchase #7	$193.46	3.845%	$200.8985	$200.90	$200.90	$200.90
7 purchase totals =	$923.63		$990.0833	$990.08	$990.10	$990.10