I don't think it's possible to always know when you'll be coming off... Just last week, i was gym leading on a vertical-slightly overhung route, and got up to where the last bolt was near my feet. The clips had all been to the left of the route at that point, so the rope was hanging down my left side, vs. between my legs. The route then went left around a slanting arete (less than vertical), and i couldn't see that the only way to stay on was to step out onto an insecure slippery foothold around the corner until I was already there...and the rope behind my left ankle. When my foot blew, i inverted, but luckily my belayer caught me before hitting anything. Only injury was a rope burn around the leg. But it got me thinking hard about the helmet--there's NEVER a good reason not to wear it outside (except when your head won't fit through a squeeze chimney, in which case I start to ask myself if I should be there in the first place... )