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	$13.65	12.639%	$15.3752	$15.38	$15.40	$15.40
Purchase #2	$169.67	5.140%	$178.391	$178.39	$178.40	$178.40
Purchase #3	$213.59	6.762%	$228.033	$228.03	$228.05	$228.00
Purchase #4	$22.55	11.847%	$25.2215	$25.22	$25.20	$25.20
Purchase #5	$82.27	9.176%	$89.8191	$89.82	$89.80	$89.80
Purchase #6	$34.27	10.145%	$37.7467	$37.75	$37.75	$37.70
Purchase #7	$59.79	8.585%	$64.923	$64.92	$64.90	$64.90
7 purchase totals =	$595.79		$639.5095	$639.51	$639.50	$639.40