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	$36.93	9.606%	$40.4775	$40.48	$40.50	$40.50
Purchase #2	$42.50	8.296%	$46.0258	$46.03	$46.05	$46.00
Purchase #3	$17.29	7.677%	$18.6174	$18.62	$18.60	$18.60
Purchase #4	$178.57	4.586%	$186.7592	$186.76	$186.75	$186.80
Purchase #5	$56.90	4.381%	$59.3928	$59.39	$59.40	$59.40
Purchase #6	$149.90	8.623%	$162.8259	$162.83	$162.85	$162.80
Purchase #7	$110.97	13.482%	$125.931	$125.93	$125.95	$125.90
7 purchase totals =	$593.06		$640.0296	$640.04	$640.10	$640.00