Use of the granny gear is just about acceptable off road on a mountain bike, although I noticed at a couple of MTB races that some cyclists used compact doubles. I believe the granny ring is also acceptable in the final stages of a ridiculously long audax. In fact, you may even gain added kudos as obviously you'd only resort to it when completely exhausted. It probably helps the effect if you have a big ring, slightly smaller ring and a much smaller bale out ring. Also acceptable would be something like a 52/39/36 and a straight through cassette on the back, as this gives the impression that you are thinking about chain line efficiency, and not just being a wuss. Using an equally spaced triple, such as a 50/40/30, especially if you also have big dinner plate gears on the back, marks you out as an amateur, and the sort of person who would use a granny ring because they have no male pride or don't know any better.