Don't sweat it too much. I run 170s on my track bike. I have long legs, and my fitter said it works for me. (Also, I got a deal on an SRM with 170s). I will say, I do have to be careful to not go too slow on a 250 track, cause I have tapped a pedal.
I previously had 167.5. I really didn't notice a difference in cranklength when going between various bikes.