Except in a few circumstances it's generally a good idea to render before exporting. Apple recommends you render all media to full. Unrendered effects that slow down the export process can cause issues such as corrupted frames. It's also a good idea to mixdown the audio before exporting.
All the best,
Class on Demand DVDs "Complete Training for FCP7," "Basic Training for FCS" and "Final Cut Express Made Easy"
Author: "Final Cut Pro 5 Editing Essentials" and "Final Cut Express 4 Editing Workshop"