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	$27.81	7.206%	$29.814	$29.81	$29.80	$29.80
Purchase #2	$115.34	4.664%	$120.7195	$120.72	$120.70	$120.70
Purchase #3	$24.86	10.455%	$27.4591	$27.46	$27.45	$27.50
Purchase #4	$161.45	11.234%	$179.5873	$179.59	$179.60	$179.60
Purchase #5	$72.56	13.806%	$82.5776	$82.58	$82.60	$82.60
Purchase #6	$128.69	7.522%	$138.3701	$138.37	$138.35	$138.40
Purchase #7	$87.85	3.683%	$91.0855	$91.09	$91.10	$91.10
7 purchase totals =	$618.56		$669.6131	$669.62	$669.60	$669.70