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	$36.90	8.267%	$39.9505	$39.95	$39.95	$40.00
Purchase #2	$66.98	8.605%	$72.7436	$72.74	$72.75	$72.70
Purchase #3	$23.38	10.569%	$25.851	$25.85	$25.85	$25.90
Purchase #4	$149.69	4.302%	$156.1297	$156.13	$156.15	$156.10
Purchase #5	$185.59	12.146%	$208.1318	$208.13	$208.15	$208.10
Purchase #6	$43.66	7.556%	$46.9589	$46.96	$46.95	$47.00
Purchase #7	$82.84	10.752%	$91.747	$91.75	$91.75	$91.70
7 purchase totals =	$589.04		$641.5125	$641.51	$641.55	$641.50