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	$18.99	7.744%	$20.4606	$20.46	$20.45	$20.50
Purchase #2	$231.85	10.102%	$255.2715	$255.27	$255.25	$255.30
Purchase #3	$96.70	13.125%	$109.3919	$109.39	$109.40	$109.40
Purchase #4	$35.37	13.372%	$40.0997	$40.10	$40.10	$40.10
Purchase #5	$148.71	13.637%	$168.9896	$168.99	$169.00	$169.00
Purchase #6	$238.82	6.419%	$254.1499	$254.15	$254.15	$254.10
Purchase #7	$113.90	3.774%	$118.1986	$118.20	$118.20	$118.20
7 purchase totals =	$884.34		$966.5618	$966.56	$966.55	$966.60