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	$25.84	9.684%	$28.3423	$28.34	$28.35	$28.30
Purchase #2	$150.53	11.151%	$167.3156	$167.32	$167.30	$167.30
Purchase #3	$175.33	6.375%	$186.5073	$186.51	$186.50	$186.50
Purchase #4	$47.96	12.189%	$53.8058	$53.81	$53.80	$53.80
Purchase #5	$248.35	12.766%	$280.0544	$280.05	$280.05	$280.10
Purchase #6	$42.84	9.210%	$46.7856	$46.79	$46.80	$46.80
Purchase #7	$71.54	6.501%	$76.1908	$76.19	$76.20	$76.20
7 purchase totals =	$762.39		$839.0018	$839.01	$839.00	$839.00