I regularly shoot my RB67+90mm at 1/15 with no problems and I also have no qualms shooting it at 1/8, but I do have to be extra careful at 1/8. My negs are sharp every time. I don't understand people who say that they're afraid to go below 1/60 or even 1/125.
EDIT: This is all handheld of course!
It all depends on physical ability. Some people are able to hold a camera still, and others are incapable of it. I cannot hand hold a 50mm lens at 1/60th of a second and get a sharp picture for the life of me. That's with a Leica M2. So it isn't as simple as just doing it. You have to be physically blessed.
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh