It's always a risk to not have a ticket, but as tourists we wouldn't chance it. I've done fairly long trips in the UK where I never had to show my ticket, but I still had one. For subways I'm used to the London one where there are barriers everywhere, but I've been to a few places that have less.
!BEER
Cheers Steve, and Thanks!
Indeed, in that way vienna has really 'public' transportation :D
I remember a Bass player i used to play with. He notoriously had no ticket. So once in a while he'd show up for rehearsals, covered in sweat from running from inspectors. Good ol' times, lol.