SInce FCPx cannot convert R3D files, you have to use RCX to create ProRes files. This is what Optimized media means. If you use RCX to create the ProRes files to 4:2:2 (or 4:4:4) then you do not need to use the Optimize option when importing. This Optimization is for when you ingest material that is not ProRes, like MTS or H264.
If you do go the 4:4:4 route, this can be very sluggish on your computer, so you can have FCPx create proxy media on ingest. They're pretty decent ProRes files perfect for editing. At any time you can turn off the proxy option to automatically see the highres 4:2:2 or 4:4:4 clips in the timeline. If you are not using 4:4:4 files your system may be fine with not creating Proxy files either.