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	$16.83	11.234%	$18.7207	$18.72	$18.70	$18.70
Purchase #2	$93.73	3.343%	$96.8634	$96.86	$96.85	$96.90
Purchase #3	$27.45	13.510%	$31.1585	$31.16	$31.15	$31.20
Purchase #4	$20.47	3.568%	$21.2004	$21.20	$21.20	$21.20
Purchase #5	$250.62	8.769%	$272.5969	$272.60	$272.60	$272.60
Purchase #6	$59.88	12.493%	$67.3608	$67.36	$67.35	$67.40
Purchase #7	$117.92	11.592%	$131.5893	$131.59	$131.60	$131.60
7 purchase totals =	$586.90		$639.4900	$639.49	$639.45	$639.60