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	$19.89	7.622%	$21.406	$21.41	$21.40	$21.40
Purchase #2	$123.86	10.386%	$136.7241	$136.72	$136.70	$136.70
Purchase #3	$227.50	9.716%	$249.6039	$249.60	$249.60	$249.60
Purchase #4	$152.76	10.552%	$168.8792	$168.88	$168.90	$168.90
Purchase #5	$114.56	10.460%	$126.543	$126.54	$126.55	$126.50
Purchase #6	$183.44	5.416%	$193.3751	$193.38	$193.40	$193.40
Purchase #7	$96.74	4.269%	$100.8698	$100.87	$100.85	$100.90
7 purchase totals =	$918.75		$997.4011	$997.40	$997.40	$997.40