The dropShadowVisible CSS property has no effect on the mx:ToolTip. Example:
<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx">
<fx:Style>
@namespace s "library://ns.adobe.com/flex/spark";
@namespace mx "library://ns.adobe.com/flex/mx";
mx|ToolTip {
dropShadowVisible: false;
}
</fx:Style>
<s:Label text="Roll over to view tooltip"
toolTip="As you can see the drop shadow is still visible"
verticalCenter="0"
horizontalCenter="0"/>
</s:Application>