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	$75.43	6.695%	$80.48	$80.48	$80.50	$80.50
Purchase #2	$109.66	11.709%	$122.5001	$122.50	$122.50	$122.50
Purchase #3	$184.86	6.518%	$196.9092	$196.91	$196.90	$196.90
Purchase #4	$216.77	5.576%	$228.8571	$228.86	$228.85	$228.90
Purchase #5	$39.73	8.175%	$42.9779	$42.98	$43.00	$43.00
Purchase #6	$36.30	10.872%	$40.2465	$40.25	$40.25	$40.20
Purchase #7	$248.43	12.122%	$278.5447	$278.54	$278.55	$278.50
7 purchase totals =	$911.18		$990.5155	$990.52	$990.55	$990.50