On my recent trip we had lots of steep climbs. I took a super lightweight bike with a compact double giving me 34-30 as the lowest gear. My normal lightweight tourer has 28-28 and I found it annoyingly low --like I was spinning and going nowhere. I guess it's all personal opinion. Sheldon used to describe gear inches into the low teens which, to me, seems like "slower than just walking." I found I really liked the 29 gear inch on my trip as a good bottom. I will admit to hopping off at a hill in Galena, IL that seemed more like a tilted wall than a hill (turned out our maps were in error and it was a useless climb, BTW).
I have no problem walking on super steep pitches. I have had back issues in the past. When the climb starts to feel like a hundred lifts a minute (pulling up on the handlebars to effect a good power stroke on the pedals), I hop off to protect the back. I find that walking shifts the exertion from the back and quads to the hammies. But this is all just my opinion and situation.