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	$77.54	7.295%	$83.1965	$83.20	$83.20	$83.20
Purchase #2	$18.45	13.829%	$21.0015	$21.00	$21.00	$21.00
Purchase #3	$64.42	11.781%	$72.0093	$72.01	$72.00	$72.00
Purchase #4	$80.73	11.757%	$90.2214	$90.22	$90.20	$90.20
Purchase #5	$93.69	12.677%	$105.5671	$105.57	$105.55	$105.60
Purchase #6	$131.48	4.431%	$137.3059	$137.31	$137.30	$137.30
Purchase #7	$137.25	9.670%	$150.5221	$150.52	$150.50	$150.50
7 purchase totals =	$603.56		$659.8238	$659.83	$659.75	$659.80