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	$139.79	12.744%	$157.6048	$157.60	$157.60	$157.60
Purchase #2	$246.63	13.420%	$279.7277	$279.73	$279.75	$279.70
Purchase #3	$131.78	8.405%	$142.8561	$142.86	$142.85	$142.90
Purchase #4	$73.89	11.226%	$82.1849	$82.18	$82.20	$82.20
Purchase #5	$68.25	5.638%	$72.0979	$72.10	$72.10	$72.10
Purchase #6	$195.79	11.531%	$218.3665	$218.37	$218.35	$218.40
Purchase #7	$233.36	13.398%	$264.6256	$264.63	$264.65	$264.60
7 purchase totals =	$1,089.49		$1,217.4635	$1,217.47	$1,217.50	$1,217.50