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	$79.90	12.569%	$89.9426	$89.94	$89.95	$89.90
Purchase #2	$172.46	6.283%	$183.2957	$183.30	$183.30	$183.30
Purchase #3	$177.60	10.853%	$196.8749	$196.87	$196.85	$196.90
Purchase #4	$46.65	11.345%	$51.9424	$51.94	$51.95	$51.90
Purchase #5	$128.43	6.638%	$136.9552	$136.96	$136.95	$137.00
Purchase #6	$42.62	12.365%	$47.89	$47.89	$47.90	$47.90
Purchase #7	$73.90	12.740%	$83.3149	$83.31	$83.30	$83.30
7 purchase totals =	$721.56		$790.2157	$790.21	$790.20	$790.20