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	$17.97	12.766%	$20.2641	$20.26	$20.25	$20.30
Purchase #2	$139.52	4.697%	$146.0733	$146.07	$146.05	$146.10
Purchase #3	$249.61	4.287%	$260.3108	$260.31	$260.30	$260.30
Purchase #4	$100.62	9.228%	$109.9052	$109.91	$109.90	$109.90
Purchase #5	$177.33	12.571%	$199.6222	$199.62	$199.60	$199.60
Purchase #6	$15.70	13.798%	$17.8663	$17.87	$17.85	$17.90
Purchase #7	$148.72	13.173%	$168.3109	$168.31	$168.30	$168.30
7 purchase totals =	$849.47		$922.3528	$922.35	$922.25	$922.40