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	$19.34	6.436%	$20.5847	$20.58	$20.60	$20.60
Purchase #2	$139.78	7.870%	$150.7807	$150.78	$150.80	$150.80
Purchase #3	$184.32	6.703%	$196.675	$196.68	$196.70	$196.70
Purchase #4	$239.74	10.337%	$264.5219	$264.52	$264.50	$264.50
Purchase #5	$35.52	8.389%	$38.4998	$38.50	$38.50	$38.50
Purchase #6	$152.92	6.183%	$162.375	$162.38	$162.40	$162.40
Purchase #7	$137.96	5.509%	$145.5602	$145.56	$145.55	$145.60
7 purchase totals =	$909.58		$978.9973	$979.00	$979.05	$979.10