The requirement to render is more related to the performance of your computer than it is a particular file format (with some exceptions). If you are importing media and optimizing that media, on a sufficiently equipped computer you should not need to render those clips when dropped into the timeline. If you apply effects and transitions, etc., then yes, you will see the orange render bar appear above those clips. You still don't need to render those unless they are not playing back to your satisfaction. Many users don not render at all. Upon exporting a project, fcpx will automatically render what is required.
You may want to be sure you turn OFF background render in the fcpx preferences so it won't attempt to do this each time you apply an effect or color correction. Just manually select render on the clip(s) whenever you think you need it.
Here is a list of the Apple supported image formats:
help.apple.com/finalcutpro/mac/10.0.4/#ver2833f855 
