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	$6.97	13.160%	$7.8873	$7.89	$7.90	$7.90
Purchase #2	$137.37	5.239%	$144.5668	$144.57	$144.55	$144.60
Purchase #3	$212.75	6.844%	$227.3106	$227.31	$227.30	$227.30
Purchase #4	$12.59	5.648%	$13.3011	$13.30	$13.30	$13.30
Purchase #5	$14.88	8.745%	$16.1813	$16.18	$16.20	$16.20
Purchase #6	$99.86	9.553%	$109.3996	$109.40	$109.40	$109.40
Purchase #7	$34.29	13.220%	$38.8231	$38.82	$38.80	$38.80
7 purchase totals =	$518.71		$557.4698	$557.47	$557.45	$557.50