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	$248.34	12.527%	$279.4496	$279.45	$279.45	$279.40
Purchase #2	$7.63	9.282%	$8.3382	$8.34	$8.35	$8.30
Purchase #3	$196.47	6.376%	$208.9969	$209.00	$209.00	$209.00
Purchase #4	$128.72	6.628%	$137.2516	$137.25	$137.25	$137.30
Purchase #5	$120.87	8.516%	$131.1633	$131.16	$131.15	$131.20
Purchase #6	$37.59	12.513%	$42.2936	$42.29	$42.30	$42.30
Purchase #7	$82.49	9.812%	$90.5839	$90.58	$90.60	$90.60
7 purchase totals =	$822.11		$898.0771	$898.07	$898.10	$898.10