Defragging all my drives but c:\ monthly
( reason why not c:\ is cos its fat32, wich takes way too long to defrag, still hardly was at 10% after 20 hours of defragging so i gave up defragging on that drive, result is the avrg true read of files (not just ramdom data) is just 10 mb compared to the disks usual 50 ish mb )
As for all others, ntfs drives, including my 500 gb raid 0, are defragged regurally, quick 10 minutes job but elimates a lot of fragments as my data is always on the move, always downloading, burning and deleting a lot of data.
Just defrag it if analization gives some red spots, if not just leave it if its all blue...
It doesn't matter if its a raid array or a single disk, defragging is same...
If you want to keep your hdd as fast s you can you should defrag it at leats once a month, I sometimes do it once per 2 days manually and automated it happens daily when my screensaver is on ( using diskkeeper).