After reading Sheldon's article on crank length, I got the idea in my head that longer can be bad but there's no downside to shorter arms.
http://sheldonbrown.com/cranks.html
On that note, all my road bikes have 170s (I'm 5' 9") and all my mountain bikes have 170s or 175s. Honestly, I don't think it's a huge deal because I've never been able to tell the difference from one bike to the next. I would like to try some 165s though for my next fixed gear build...