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	$18.66	10.387%	$20.5982	$20.60	$20.60	$20.60
Purchase #2	$95.91	12.654%	$108.0465	$108.05	$108.05	$108.00
Purchase #3	$134.61	13.108%	$152.2547	$152.25	$152.25	$152.30
Purchase #4	$100.48	7.330%	$107.8452	$107.85	$107.85	$107.80
Purchase #5	$26.73	9.844%	$29.3613	$29.36	$29.35	$29.40
Purchase #6	$185.64	12.812%	$209.4242	$209.42	$209.40	$209.40
Purchase #7	$29.53	10.370%	$32.5923	$32.59	$32.60	$32.60
7 purchase totals =	$591.56		$660.1224	$660.12	$660.10	$660.10