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	$36.54	9.835%	$40.1337	$40.13	$40.15	$40.10
Purchase #2	$95.89	7.805%	$103.3742	$103.37	$103.35	$103.40
Purchase #3	$122.26	13.262%	$138.4741	$138.47	$138.45	$138.50
Purchase #4	$90.98	5.516%	$95.9985	$96.00	$96.00	$96.00
Purchase #5	$156.45	10.243%	$172.4752	$172.48	$172.50	$172.50
Purchase #6	$33.29	8.773%	$36.2105	$36.21	$36.20	$36.20
Purchase #7	$70.54	4.204%	$73.5055	$73.51	$73.50	$73.50
7 purchase totals =	$605.95		$660.1717	$660.17	$660.15	$660.20