I have never run anything but regular in any ATV that I've ever owned. I like to keep my ATV's stock, though, if I can because I am looking for reliability more than performance. I mostly ride utility ATV's, but I do have one sport quad. I believe the further from stock you get, the more unreliable they are. I often ride on trails where there is nothing or no one around for miles. I wouldn't want to have to walk that far.
I have heard a lot of other people say they run premium.