when I was 16 and racing i rode on 170 mm. In my mid 20's, most of my riding was recreational and I changed to 175 mm. I came back to the sport at 52 and started using my old 175 mm. Due to an accident I had to get another bike, couldn't find replacement parts for my old bike so I got a bike with 172.5 mm.
(my inseam is about 36 inches, 6 ft 1 inch height).
A few observations:
Although the absolute differences are small, 2.5 and 5 mm, they seem to be significant.
I felt that my climbing was better with 175 mm.
Spinning was better with 170 mm
Although I sometimes think about getting 175 mm (hill climbing is my preferred activity), the 172.5 is a good compromise for me, allowing comfortable riding on the flats and sufficient leverage in the hills.
Your riding style, (slow vs fast cadence; ankle angle and movement, etc,), abilities (hill climber or sprinter), and femur length (http://www.nettally.com/palmk/crderiva.html
) will dictate your optimal crank length.