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	$17.79	11.754%	$19.881	$19.88	$19.90	$19.90
Purchase #2	$129.47	9.510%	$141.7826	$141.78	$141.80	$141.80
Purchase #3	$91.30	9.590%	$100.0557	$100.06	$100.05	$100.10
Purchase #4	$121.95	13.112%	$137.9401	$137.94	$137.95	$137.90
Purchase #5	$85.84	7.551%	$92.3218	$92.32	$92.30	$92.30
Purchase #6	$88.89	8.571%	$96.5088	$96.51	$96.50	$96.50
Purchase #7	$26.55	11.144%	$29.5087	$29.51	$29.50	$29.50
7 purchase totals =	$561.79		$617.9987	$618.00	$618.00	$618.00