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	$35.54	12.500%	$39.9825	$39.98	$40.00	$40.00
Purchase #2	$42.75	8.260%	$46.2812	$46.28	$46.30	$46.30
Purchase #3	$35.73	13.107%	$40.4131	$40.41	$40.40	$40.40
Purchase #4	$82.36	3.585%	$85.3126	$85.31	$85.30	$85.30
Purchase #5	$201.94	6.210%	$214.4805	$214.48	$214.50	$214.50
Purchase #6	$28.43	11.207%	$31.6162	$31.62	$31.60	$31.60
Purchase #7	$98.28	10.383%	$108.4844	$108.48	$108.50	$108.50
7 purchase totals =	$525.03		$566.5705	$566.56	$566.60	$566.60