Had an interesting discussion yesterday regarding crank length: Basically, one of the guys at the Specialized Concept Store said that he felt that all things considered, crank length was probably one of the last things to change, and possibly wouldn't need changing at all.
I'm going to get a bike fit done at Stows: That should then allow me to see if there are any fit issues which would be fixed by changing crank length: Otherwise I'm sticking to 170mm.
Crank length made a huge difference for me. I went from 172.5mm to 170mm, the LBS thought I was mad as it cost quite a bit, but the difference really was amazing.
I used to get rock around a lot, and my thighs were always aching. Crank length seemed to solve it.
What sort of bike fit are you having done?