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	$83.59	7.722%	$90.0448	$90.04	$90.05	$90.00
Purchase #2	$102.57	10.572%	$113.4137	$113.41	$113.40	$113.40
Purchase #3	$170.85	6.621%	$182.162	$182.16	$182.15	$182.20
Purchase #4	$81.71	7.861%	$88.1332	$88.13	$88.15	$88.10
Purchase #5	$120.58	11.604%	$134.5721	$134.57	$134.55	$134.60
Purchase #6	$52.86	10.329%	$58.3199	$58.32	$58.30	$58.30
Purchase #7	$163.85	12.660%	$184.5934	$184.59	$184.60	$184.60
7 purchase totals =	$776.01		$851.2391	$851.22	$851.20	$851.20