In my own unscientific tests riding up a short 30 degree slope on my MTBs, I've still yet to conquer it on 170s even though I never felt like I was running out of gear necessarily. It's a piece of cake to crawl up it on 175s, however.

Maybe someday I'll get some 165 road cranks and do some testing on steep asphalt with those and my current 172.5s. 39x28 on the short cranks, 39x25 on the long cranks.