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	$28.69	11.131%	$31.8835	$31.88	$31.90	$31.90
Purchase #2	$19.72	8.121%	$21.3215	$21.32	$21.30	$21.30
Purchase #3	$5.98	4.511%	$6.2498	$6.25	$6.25	$6.20
Purchase #4	$155.70	5.469%	$164.2152	$164.22	$164.20	$164.20
Purchase #5	$123.83	6.192%	$131.4976	$131.50	$131.50	$131.50
Purchase #6	$113.59	13.798%	$129.2631	$129.26	$129.25	$129.30
Purchase #7	$144.60	7.588%	$155.5722	$155.57	$155.55	$155.60
7 purchase totals =	$592.11		$640.0029	$640.00	$639.95	$640.00