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	$18.93	12.108%	$21.222	$21.22	$21.20	$21.20
Purchase #2	$81.53	10.201%	$89.8469	$89.85	$89.85	$89.80
Purchase #3	$209.38	11.372%	$233.1907	$233.19	$233.20	$233.20
Purchase #4	$167.60	7.565%	$180.2789	$180.28	$180.30	$180.30
Purchase #5	$43.39	12.392%	$48.7669	$48.77	$48.75	$48.80
Purchase #6	$98.98	13.487%	$112.3294	$112.33	$112.35	$112.30
Purchase #7	$211.88	10.305%	$233.7142	$233.71	$233.70	$233.70
7 purchase totals =	$831.69		$919.3490	$919.35	$919.35	$919.30