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	$6.45	5.177%	$6.7839	$6.78	$6.80	$6.80
Purchase #2	$73.95	6.675%	$78.8862	$78.89	$78.90	$78.90
Purchase #3	$40.93	3.795%	$42.4833	$42.48	$42.50	$42.50
Purchase #4	$72.59	3.344%	$75.0174	$75.02	$75.00	$75.00
Purchase #5	$124.59	11.856%	$139.3614	$139.36	$139.35	$139.40
Purchase #6	$150.94	6.261%	$160.3904	$160.39	$160.40	$160.40
Purchase #7	$86.95	4.382%	$90.7601	$90.76	$90.75	$90.80
7 purchase totals =	$556.40		$593.6827	$593.68	$593.70	$593.80