Either one of those is the theory, but in truth, few people do. It's even harder when you consider that, really, to get the best support and reliability you should be buying disks from your server / SAN manufacturer. So, for example, we put in a HP SAN last week with about 70 disks - we pretty much have to buy them from HP, so you're kind of stuck there.
It's just one of those things to be aware of, and a good reason to avoid RAID5 at nearly all costs.
Ahh, yes, I seem to recall that that's where I was heading...except that no one who pontificates over these things on t'webz actually makes a hard-and-fast recommendation that those of us with goldfish-like attention spans can follow.
Until now...until /YOU/!