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	$19.55	8.845%	$21.2792	$21.28	$21.30	$21.30
Purchase #2	$110.38	6.706%	$117.7821	$117.78	$117.80	$117.80
Purchase #3	$64.37	13.118%	$72.8141	$72.81	$72.80	$72.80
Purchase #4	$36.25	12.732%	$40.8654	$40.87	$40.85	$40.90
Purchase #5	$17.48	3.817%	$18.1472	$18.15	$18.15	$18.10
Purchase #6	$55.66	8.125%	$60.1824	$60.18	$60.20	$60.20
Purchase #7	$186.85	13.718%	$212.4821	$212.48	$212.50	$212.50
7 purchase totals =	$490.54		$543.5525	$543.55	$543.60	$543.60