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	$35.80	11.689%	$39.9847	$39.98	$40.00	$40.00
Purchase #2	$80.66	3.545%	$83.5194	$83.52	$83.50	$83.50
Purchase #3	$40.41	12.701%	$45.5425	$45.54	$45.55	$45.50
Purchase #4	$11.43	12.722%	$12.8841	$12.88	$12.90	$12.90
Purchase #5	$202.34	13.192%	$229.0327	$229.03	$229.05	$229.00
Purchase #6	$125.34	5.669%	$132.4455	$132.45	$132.45	$132.40
Purchase #7	$134.46	7.563%	$144.6292	$144.63	$144.65	$144.60
7 purchase totals =	$630.44		$688.0381	$688.03	$688.10	$687.90