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	$38.27	12.302%	$42.978	$42.98	$43.00	$43.00
Purchase #2	$139.55	5.504%	$147.2308	$147.23	$147.25	$147.20
Purchase #3	$61.45	4.610%	$64.2828	$64.28	$64.30	$64.30
Purchase #4	$114.82	4.843%	$120.3807	$120.38	$120.40	$120.40
Purchase #5	$35.78	6.262%	$38.0205	$38.02	$38.00	$38.00
Purchase #6	$45.48	5.825%	$48.1292	$48.13	$48.15	$48.10
Purchase #7	$75.81	3.555%	$78.505	$78.51	$78.50	$78.50
7 purchase totals =	$511.16		$539.5270	$539.53	$539.60	$539.50