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	$35.83	8.783%	$38.9769	$38.98	$39.00	$39.00
Purchase #2	$213.42	10.547%	$235.9294	$235.93	$235.95	$235.90
Purchase #3	$35.95	13.862%	$40.9334	$40.93	$40.95	$40.90
Purchase #4	$29.50	6.861%	$31.524	$31.52	$31.50	$31.50
Purchase #5	$168.50	9.335%	$184.2295	$184.23	$184.25	$184.20
Purchase #6	$175.93	7.371%	$188.8978	$188.90	$188.90	$188.90
Purchase #7	$121.39	8.385%	$131.5686	$131.57	$131.55	$131.60
7 purchase totals =	$780.52		$852.0596	$852.06	$852.10	$852.00