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	$18.80	7.869%	$20.2794	$20.28	$20.30	$20.30
Purchase #2	$46.67	13.247%	$52.8524	$52.85	$52.85	$52.90
Purchase #3	$85.44	4.605%	$89.3745	$89.37	$89.35	$89.40
Purchase #4	$156.41	10.839%	$173.3633	$173.36	$173.35	$173.40
Purchase #5	$246.45	13.749%	$280.3344	$280.33	$280.35	$280.30
Purchase #6	$83.75	4.686%	$87.6745	$87.67	$87.65	$87.70
Purchase #7	$77.77	10.599%	$86.0128	$86.01	$86.00	$86.00
7 purchase totals =	$715.29		$789.8913	$789.87	$789.85	$790.00