I work for a major hosting company, and the most common thing I see in my enterprise segment (not necessarily that I recommend, but what I see), assuming that the server is standalone / using local storage, is a RAID 1 OS array, and a RAID5 data array.
Now, as hard drives get bigger, RAID 5 really becomes less ideal, as your likelihood of hitting a URE during a single-drive rebuild is pretty high.
But since you seem to be specifically asking about the OS drive, yes, RAID 1 is standard and usually sufficient, unless you're going to be running other apps off that drive.
