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	$34.69	11.367%	$38.6332	$38.63	$38.65	$38.60
Purchase #2	$227.27	9.265%	$248.3266	$248.33	$248.35	$248.30
Purchase #3	$73.43	12.141%	$82.3451	$82.35	$82.35	$82.30
Purchase #4	$77.81	10.570%	$86.0345	$86.03	$86.05	$86.00
Purchase #5	$78.39	8.318%	$84.9105	$84.91	$84.90	$84.90
Purchase #6	$86.27	4.216%	$89.9071	$89.91	$89.90	$89.90
Purchase #7	$7.99	11.260%	$8.8897	$8.89	$8.90	$8.90
7 purchase totals =	$585.85		$639.0467	$639.05	$639.10	$638.90