I think you should use the longest cranks that you can without causing knee problems. If I used 180mm crank arms, I think I would have knee problems. Most of my bikes have 175mm, but I put 170mm arms on one bike recently and I can just barely perceive less stress on my knees after riding that bike than I feel after riding my other bikes for long rides.

In this case I define perceived stress on my knees as how they feel walking up stairs after the bike ride.