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	$5.89	12.415%	$6.6212	$6.62	$6.60	$6.60
Purchase #2	$87.25	8.164%	$94.3731	$94.37	$94.35	$94.40
Purchase #3	$90.52	13.179%	$102.4496	$102.45	$102.45	$102.40
Purchase #4	$73.89	7.718%	$79.5928	$79.59	$79.60	$79.60
Purchase #5	$84.91	6.529%	$90.4538	$90.45	$90.45	$90.50
Purchase #6	$248.38	5.705%	$262.5501	$262.55	$262.55	$262.60
Purchase #7	$122.81	12.315%	$137.9341	$137.93	$137.95	$137.90
7 purchase totals =	$713.65		$773.9747	$773.96	$773.95	$774.00