There are other designs but it's what's used commonly that's being discussed, I can't recall ever having other than straight tapered bolts/nuts, whether for steel or alloy, car or caravan.
It would though seem 17mm is now very common judging by the ratio of those for sale on the Internet.
Thinking about it my cheap wrench like many has a duel 17/19mm socket so it matters not what the head size is, more importantly though, the taper underneath will still be the same size regardless of the head size.