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	$18.82	12.140%	$21.1047	$21.10	$21.10	$21.10
Purchase #2	$21.89	7.841%	$23.6064	$23.61	$23.60	$23.60
Purchase #3	$244.88	12.316%	$275.0394	$275.04	$275.05	$275.00
Purchase #4	$71.29	10.238%	$78.5887	$78.59	$78.60	$78.60
Purchase #5	$8.32	8.391%	$9.0181	$9.02	$9.00	$9.00
Purchase #6	$98.49	6.452%	$104.8446	$104.84	$104.85	$104.80
Purchase #7	$134.37	12.331%	$150.9392	$150.94	$150.95	$150.90
7 purchase totals =	$598.06		$663.1411	$663.14	$663.15	$663.00