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	$87.76	13.482%	$99.5918	$99.59	$99.60	$99.60
Purchase #2	$153.42	9.610%	$168.1637	$168.16	$168.15	$168.20
Purchase #3	$217.53	4.186%	$226.6358	$226.64	$226.65	$226.60
Purchase #4	$131.94	4.470%	$137.8377	$137.84	$137.85	$137.80
Purchase #5	$180.37	3.618%	$186.8958	$186.90	$186.90	$186.90
Purchase #6	$198.73	3.587%	$205.8584	$205.86	$205.85	$205.90
Purchase #7	$246.78	3.383%	$255.1286	$255.13	$255.15	$255.10
7 purchase totals =	$1,216.53		$1,280.1118	$1,280.12	$1,280.15	$1,280.10