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	$20.39	4.236%	$21.2537	$21.25	$21.25	$21.30
Purchase #2	$79.85	12.855%	$90.1147	$90.11	$90.10	$90.10
Purchase #3	$115.38	8.397%	$125.0685	$125.07	$125.05	$125.10
Purchase #4	$29.37	7.527%	$31.5807	$31.58	$31.60	$31.60
Purchase #5	$23.64	12.215%	$26.5276	$26.53	$26.55	$26.50
Purchase #6	$190.25	5.377%	$200.4797	$200.48	$200.50	$200.50
Purchase #7	$37.57	12.208%	$42.1565	$42.16	$42.15	$42.20
7 purchase totals =	$496.45		$537.1814	$537.18	$537.20	$537.30