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.