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	$24.54	13.257%	$27.7933	$27.79	$27.80	$27.80
Purchase #2	$95.78	9.805%	$105.1712	$105.17	$105.15	$105.20
Purchase #3	$105.25	12.542%	$118.4505	$118.45	$118.45	$118.50
Purchase #4	$131.75	4.608%	$137.821	$137.82	$137.80	$137.80
Purchase #5	$248.71	12.732%	$280.3758	$280.38	$280.40	$280.40
Purchase #6	$56.38	7.834%	$60.7968	$60.80	$60.80	$60.80
Purchase #7	$109.55	4.584%	$114.5718	$114.57	$114.55	$114.60
7 purchase totals =	$771.96		$844.9804	$844.98	$844.95	$845.10