Crank length is more personal than mathematical, meaning a formula on it's own is not necessarily the answer. From track racing, a longer crank allows you to put the power down easier with a higher gear. It's one for the powerful guys that don't like to spin so much. Spinners tend to like shorter cranks as your feet/pedals don't have to travel in such a large circle. It's not really dependant on height, but personal preference relating to how you like to pedal. Apply that philosophy to your riding.
The main reason that I have heard of tall guys wanting longer cranks is to make climbing hills that little bit easier. In my experience, aside from the very tall, that is the only reason I have seen for people wanting extra long cranks.
For me, I race track on 170s at 6'5", and have 172.5 on my road bike. Both cranks came with second hand bikes, so weren't my choosing, but I must say I can't fault them. Having said that, I'm looking at a new Cervelo R3 when I can finally scrape the money together, and am going for a Rival drivetrain because I want to try out their 180mm crankset. I was looking at an Ultegra model as I have that now and like it, but the upgrade to a DA crankset in 180mm is a bit pricey.