Car free since 1995
Join Date: Sep 2006
Bikes: Birdy Yellow, Birdy Monocoque
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That twitchy feel will disappear after the first ride, and you will be addicted to it from then on.
I put 165s on my bike because I have knee pain. The pain mostly went away, but I'm not sure whether it was that or not. Remember, we are talking about 1/2 of a centimeter here, so this isn't going to make much difference one way or the other. It also doesn't affect the gearing much (check Sheldon's gear calculator, and you will see hardly any difference between 170mm and 165mm cranks).
I'm a physician with almost zero physics, and thus can't claim any expertise here, but crank length seems to me to be a bit more of a religious argument than a scientific one. If you do some web searches, you'll see all sorts of ideas about inseam length relative to crank length.
As for me, I'm 5'10", and 165s subjectively seem about right. If you are shorter or have short legs, you might consider going lower. But then, you'll be stuck in a vicious downward cycle with your other bikes!