Depends on if you want to use hardware RAID, or software. If you have a real hardware RAID-5 controller, I'd go the hardware route. For that, you have to configure the array in BIOS, along with the online-spare drive, before you do your install. Once in the installer, you'll see one drive, the entire array you defined, not the separate disks.
If you want to go the software RAID route, you have to (obviously), install first. Depending on the distro you're using, it may give you the option of building the array there, or you may have to complete installation, then build it. IMHO, I think that hardware RAID gives you better performance.