This has so many variables it is a tough question to answer.
I believe the basic Filemaker recommendation is to advise some sort of hardware data redundancy no matter what it is. For WebDirect I believe the recommendation is for performance. Also, Filemaker sometimes has random documentation that needs correction.
I would guess 10 is recommended for WebDirect because of the faster read/write speed balance. In most cases I might have I would use 10 over 5. Disk space goes to 50% but storage space is pretty cheap these days. If your WebDirect use is minimal you may want to consider a RAID without thought of how it affects WebDirect.
I have a solution with minimal WebDirect use running on RAID 1 without issues.