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	$7.31	6.378%	$7.7762	$7.78	$7.80	$7.80
Purchase #2	$201.46	4.539%	$210.6043	$210.60	$210.60	$210.60
Purchase #3	$242.42	13.236%	$274.5067	$274.51	$274.50	$274.50
Purchase #4	$107.43	3.672%	$111.3748	$111.37	$111.35	$111.40
Purchase #5	$176.33	10.419%	$194.7018	$194.70	$194.70	$194.70
Purchase #6	$78.26	6.799%	$83.5809	$83.58	$83.60	$83.60
Purchase #7	$25.69	9.453%	$28.1185	$28.12	$28.10	$28.10
7 purchase totals =	$838.90		$910.6632	$910.66	$910.65	$910.70