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	$109.38	12.766%	$123.3435	$123.34	$123.35	$123.30
Purchase #2	$152.77	4.175%	$159.1481	$159.15	$159.15	$159.10
Purchase #3	$55.83	5.833%	$59.0866	$59.09	$59.10	$59.10
Purchase #4	$30.33	9.741%	$33.2844	$33.28	$33.30	$33.30
Purchase #5	$52.70	6.791%	$56.2789	$56.28	$56.30	$56.30
Purchase #6	$7.92	12.509%	$8.9107	$8.91	$8.90	$8.90
Purchase #7	$70.91	13.302%	$80.3424	$80.34	$80.35	$80.30
7 purchase totals =	$479.84		$520.3946	$520.39	$520.45	$520.30