According to the OP, the bike shop told him that he had 170mm cranks. If he didn't even know what length cranks he had, then i'm guessing, it's probably not gonna make a whole big difference(maybe placebo effect) if he goes 5mm smaller. I mean, there are always gonna be cork sniffing, elbow patch wearing people out there that swear by this and that, and some of it may even be proven by "men of science" who write informative articles. I used to swear by 165s, but I'm riding some 170 Campys and they feel fine, to me.