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	$8.39	10.823%	$9.298	$9.30	$9.30	$9.30
Purchase #2	$92.82	10.207%	$102.2941	$102.29	$102.30	$102.30
Purchase #3	$7.89	13.659%	$8.9677	$8.97	$8.95	$9.00
Purchase #4	$57.34	4.380%	$59.8515	$59.85	$59.85	$59.90
Purchase #5	$89.98	11.796%	$100.594	$100.59	$100.60	$100.60
Purchase #6	$155.31	10.753%	$172.0105	$172.01	$172.00	$172.00
Purchase #7	$175.30	6.538%	$186.7611	$186.76	$186.75	$186.80
7 purchase totals =	$587.03		$639.7769	$639.77	$639.75	$639.90