Your OS knows how to use the RAID, Ive always defragged my systems that were RAID'd and saw good results.
No, dont compress files, then your system has to uncompress em (CPU time, etc) to uncompress em for use.
Depending on my system usage, I tend to defrag every few days, more so when I've done a lot of file management (downloaded files, deleted stuff, etc). I just use the microsoft one