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	$20.63	3.221%	$21.2945	$21.29	$21.30	$21.30
Purchase #2	$131.96	13.736%	$150.086	$150.09	$150.10	$150.10
Purchase #3	$113.45	11.841%	$126.8836	$126.88	$126.90	$126.90
Purchase #4	$112.90	9.436%	$123.5532	$123.55	$123.55	$123.60
Purchase #5	$231.85	12.659%	$261.1999	$261.20	$261.20	$261.20
Purchase #6	$95.82	10.852%	$106.2184	$106.22	$106.20	$106.20
Purchase #7	$6.78	7.857%	$7.3127	$7.31	$7.30	$7.30
7 purchase totals =	$713.39		$796.5483	$796.54	$796.55	$796.60