I've been riding fixed with 175s for the past two decades. In that time I've had maybe two or three pedal strikes, none fatal, so I've had little motivation to try shorter cranks. I do run out of spin at about 170-180rpm, but takes me over 30mph with my current gearing so that's not a real problem either.

I have been wondering if it would reduce the stress on my knees if I ran shorter cranks. It seems like it should, but I was wondering if anyone had the experience of going down in length, say to 170, and finding it made a real difference. Saddle height is just over 81cm from the center of the BB to the top of the saddle and I was also wondering if there was a rule of thumb that translated that distance into an optimal crank length.

i run 170's and set my saddle to 81cm from my bb as well. this has always proved to be a comfortable saddle height for me. i then was borrowing a friend's bike that had 165's. going by my rule of 81cm, i set the saddle and went for a ride. my saddle height did not feel right at all. it felt too short while riding. i then measured my saddle from the center of my pedal axle with the cranks parallel to my seat tube on my 170's bike. i reset my saddle on the 165's bike and it fit much better. as for your knees, i'm not sure it would reduce stress, but with shorter cranks you are making tighter, smaller circular movements... so perhaps that would help...

as a matter of physics,
a shorter crank will give smaller radii of the stroke=less movement of the knee,
however, using a shorter crank also means more force is necessary to apply the same rotational torque to the spindle as with a longer crank,
(the moment arm is shorter, torque=Force x Distance from center)
when calculated out, those 5mm aren't too big a deal,
so if your dealing with a lot of hills it might make a difference
only thing i can say is try it out,
thats pretty much the only way to find out if its more comfortable

I'm 6'1", run my saddle at 80cm from BB, and use 170's on my fixie - I haven't used 175's on it, but I do have 175's on my mountain bike and 172.5's on 3 road bikes. Honestly, I cannot tell the difference between any of them after a couple of pedal revolutions...perhaps if you rode 30 miles on 175's then jumped directly on 170's you'd feel a difference but I'm thinking in real life you won't even notice. 180 rpms? Nice cadence; I'm still trying to top 170.